Don't stop converting subsequent chunks just because the length of one of them was 0: this can happen if the first character of a string is a NUL or if there are two (or more) NULs in it later. Simply remove the check for this and continue as usual even in this case. Also add a unit test verifying that we do translate NULs in input into NULs in output. Closes #16620. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78021 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775