Make TeX-completing-read-multiple return nil on empty input.

* multi-prompt.el (multi-prompt): Return nil on empty input.

* tex.el (TeX-completing-read-multiple): Define it so that empty
input results in nil across different emacs versions (<= 24.3
vs. later versions).

* style/biblatex.el ("biblatex"): Use
`TeX-completing-read-multiple' instead of
`completing-read-multiple'.
(LaTeX-arg-biblatex-cites): Use `TeX-completing-read-multiple' and
adapt handling of return value.

* style/pstricks.el (LaTeX-package-parameters): Adapt to
`TeX-completing-read-multiple' change.