[MWF] Use overload of Marshal.PtrToStringUni that reads up until the first null instead of explicitly specifying length

This fixes the two tests broken by 0ac61b03fcd9baf75739d3c567b36647bde08025
Merge pull request #1361 from akoeplinger/fix-forms

[MWF] Use overload of Marshal.PtrToStringUni that reads up until the first null instead of explicitly specifying length