Fix a bug with some text fields

Not sure when this happens exactly, but it is possible that
InputConnection#getTextBeforeCursor returns null. This
happens for example upon rotating the screen with the
composing field empty in Gmail.
In this case, StringBuilder#append will convert the null
pointer into the string "null", which is sure better than a
crash, but can have a number of bad side-effects, like
auto-caps not working.

Bug: 7533034
Change-Id: Ia1cfab432c13a12ff1c2f013c59bac05a587f553