Allow blank strings for the text property input

Allow blank strings for the text property input

The input-dialog for setting the text property was requiring the
string to be non-empty, or it would reject the input. That's not right
- it should be possible to set the text to blank (which is common for
text fields for example), since blank will clear the attribute.

This code was just cut & pasted from the similar code for inputting
id's where the id -should- be nonempty.

The code was also not handling the case where the user cancels the
dialog for custom string properties; we need a null check.

Change-Id: I31a2ae07b99ebc59d15b76f68c514d97294a736b