Fixes:
> (file-svg:125356): Gtk-CRITICAL **: 22:11:43.459: gtk_entry_buffer_set_text: assertion 'chars != NULL' failed
… when running file-svg-export because the default of the "title" string
argument is NULL.
I did hesitate considering this was a bug in file-svg code on the
premises that a NULL value was not valid (hence replacing it as ""), but
in the end, I went with the more flexible solution, which is that NULL
is considered equivalent to "". So GimpLabelEntry code will just
transform the NULL pointer internally to an empty string.
