ProxyFCGISetEnvIf: reject invalid invocations, streamline unsets

Neither of the following makes sense:
- ProxyFCGISetEnvIf cond !VARIABLE value
- ProxyFCGISetEnvIf cond !

Error out in these cases. Also, don't execute the (unused) replacement
expression when unsetting.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1782532 13f79535-47bb-0310-9956-ffa450edef68