Änderungen

fixed #708, String + null not supported by Jass

the workaround is to translate expressions "x+null" to just "x" when
going from IM to Jass.

Also updated interpreter to support null strings properly:
"a" + null == "a"
null + "a" == "a"
null + null == null
"" != null
 (commit: e3da02d)