Support empty prompts, have the prompt cell take up no space when...

... the prompt is an empty string.

Since we are using our contact picker in a table view where the table header is acting as a prompt, it is redundant to have one displayed by the picker itself. Just setting the prompt to an empty string did not look right, though, because the first contact cell was offset by the width of the prompt and its padding. This change makes the picker look good both with and without an internal prompt.