This commit fixes the 'text is undefined' error in gr-autocomplete
by setting a default value for text, as well as catching the case
where text is set to undefined.
This commit also disables the built-in autocomplete on the input
element in the gr-autocomplete element to avoid redundancy.
Bug: Issue 4276
Change-Id: Ia4d084068da223d10273110a402690e3b0af2e94