arcpy - How to keep a parameter blank in a script tool?

I've developed a sort of find and replace script tool that functions. But I'm not able to figure out how to find empty cells in fields and replace them.For the "Find" parameter, I have tried entering "", '', None, null, and < null >.

An example that might make this clearer. Say you had a field in a table with every row empty, and you wanted to make them say "Not Available" or something instead, which you would enter in the "replace" parameter.

I feel like this is a tool/syntax issue and not really a script one, no?

import arcpy

FC = arcpy.GetParameterAsText(0)
oldText = arcpy.GetParameterAsText(1)

replaceText = arcpy.GetParameterAsText(2)
targetField = arcpy.GetParameterAsText(3)


queryString = targetField + ' = ' + "'" + oldText + "'"
try:

    with arcpy.da.UpdateCursor(FC, (targetField), queryString) as cursor:
        for row in cursor:
            if row[0] == oldText:

                row[0] = replaceText
                cursor.updateRow(row)

                arcpy.AddMessage("String where " + queryString + " found and      replaced with " + replaceText)

    except:
        arcpy.AddMessage(arcpy.GetMessages())
        arcpy.AddError("Failed to find and replace where " + queryString)

Answer

Expanding on the comments:

import sys, arcpy
FC = arcpy.GetParameterAsText(0)
oldText = arcpy.GetParameterAsText(1)
replaceText = arcpy.GetParameterAsText(2)
targetField = arcpy.GetParameterAsText(3)

if len(replaceText) == 0:
    replaceNone = True
else:
    replaceNone = False


try:
    if replaceNone:
        d = arcpy.Describe(FC)
        # branch for each database type
        if d.dataType == "ShapeFile":
            queryString = targetField + ' = \'\'' # empty string '' or not at all
            with arcpy.da.UpdateCursor(FC, (targetField), queryString) as cursor:
                for row in cursor:
                    if len(row[0].replace(" ","")) == 0:

                        row[0] = replaceText
                        cursor.updateRow(row)
                        arcpy.AddMessage("String where none found replaced with {0}".format(replaceText))

        elif d.dataType == "FeatureClass":
            queryString = targetField + ' is null'
            with arcpy.da.UpdateCursor(FC, (targetField), queryString) as cursor:
                for row in cursor:
                    row[0] = replaceText # go ahead and replace the null fields as decided by the cursor
                    cursor.updateRow(row)

                    arcpy.AddMessage("String where none found replaced with {0}".format(replaceText))
        else:
            arcpy.AddError("Unknown feature type")
            sys.exit(-1)

    else:
        # any string that is not None
        queryString = targetField + ' = ' + "'" + oldText + "'"
        with arcpy.da.UpdateCursor(FC, (targetField), queryString) as cursor:
            for row in cursor:

                if row[0] == oldText:
                    row[0] = replaceText
                    cursor.updateRow(row)
                    arcpy.AddMessage("String where " + queryString + " found and      replaced with " + replaceText)

except:
        arcpy.AddMessage(arcpy.GetMessages())
        arcpy.AddError("Failed to find and replace where " + queryString)

Because of the different handling of empty/null cells between databases and actual values it's necessary to branch early. Different query strings are needed for null values and, as I have said on your previous post, the whereclause can be omitted as it would just confuse the issue. It's better to test for empty/whitespace strings with len( and replace(" ","") operators to clear out just space strings but to leave any genuine strings... also replace(" ","").replace("\t","") might be a good idea but I haven't seen many fields filled with tab in a while so (being lazy) I skip it; I'm sure that will come back to bite me one day.

To put to rest (conclusively) the value of an optional string argument I created a simple script:

import arcpy

Par = arcpy.GetParameterAsText(0)

if Par == None:
    arcpy.AddMessage("Parameter is none")
else:
    if len(Par) == 0:
        arcpy.AddMessage("Parameter is 0 length string")

    else:
        arcpy.AddMessage("Parameter is {0}".format(Par))

Setup like this:

and the results:

So it turns out that an optional unfilled string is a 0 length string, not None or '#', which changes the line if replaceText == None: to if len(replaceText) == 0: to test if the replaceText parameter of the tool is blank - insinuating Null, in the case where the replaceText is blank Null values in databases and empty/whitespace strings in shapefiles will be replaced. Note that this only works for strings... different strategies need to be employed with numeric and date fields, but that is a separate question by itself.