Sass compiler now treats null-valued variables as unassigned (#12430).

Change-Id: I13a11b4c7d148646ad3eb5492d9dd8d736e63f39
Merge commit '915c37b569600a8d37db2a0800a30474da49e255' into master

915c37b Sass compiler now treats null-valued variables as unassigned (#12430).

Conflicts:
	theme-compiler/src/com/vaadin/sass/internal/parser/Parser.java

Change-Id: If9789f8d0638377d27e77ec4be0fdf0f96e4b1af