env: Do not return empty tool env vars
A compiler or other tool with an empty string as name does not make
sense as it anyway can not be used and causes a failure later in
parse_entry.
Fix
@@ -340,6 +340,10 @@ This is probably wrong, it should always point to the native compiler.''' % evar
command = os.environ.get(evar)
if command is not None:
command = shlex.split(command)
+
+ # Do not return empty string entries
+ if command is not None and len(command) == 0:
+ return None
return command
class Directories:
