glsl: Don't assert when the value returned by a function has no rvalue

glsl: Don't assert when the value returned by a function has no rvalue

The rvalue of the returned value can be NULL if the shader says
'return foo();' and foo() is a function that returns void.

Existing GLSL specs do *NOT* say that this is an error.  The type of
the return value is void.  If the return type of the function is also
void, then this should compile without error.  I expect that future
versions of the GLSL spec will fix this (wink, wink, nudge, nudge).

Fixes piglit test glsl-1.10/compiler/expressions/return-01.vert and
bugzilla #33308.

NOTE: This is a candidate for the 7.9 and 7.10 branches.