Fix crash on empty CMAKE_<lang>_COMPILER value (#13901)

Since commit fd33bf93 (fix for bug 6102, allow users to change the
compiler, 2007-12-13) we keep an internal ;-list of language compiler
variable and value pairs.  Preserve empty values on expansion to ensure
that the key/value pairing remains consistent.
Merge topic 'empty-compiler-crash' into next

ccfeefa Fix crash on empty CMAKE_<lang>_COMPILER value (#13901)