HistoryCompleter: Do not overwrite text in the FancyLineEdit

Do only set the first item of the completion list if there is no
text in the line edit when the completer gets attached. Otherwise
we might overwrite default values that we actually expect to stick
around.

Task-number: QTCREATORBUG-11291
Change-Id: Iee9aa2a5475b840ede201f306e3b4164c6e88120
Reviewed-by: hjk <hjk121@nokiamail.com>