Avoid redundant set_preedit_string(null) calls
See gtk#1447 (closed), those redundant events trigger unwanted behavior when spuriously emitted (eg. after focus in), that would make sense if there was an actual preedit string change.
See gtk#1447 (closed), those redundant events trigger unwanted behavior when spuriously emitted (eg. after focus in), that would make sense if there was an actual preedit string change.