Remove `zf != NULL` check from `ZIPPacker::start_file`

This check was removed because it introduces a bug which prevents
ZIPPacker from actually adding any files, since it must be opened before
adding any files (and therefore shouldn't be NULL at the start of
`start_file`).

modules/zip/zip_packer.cpp

@@ -50,7 +50,6 @@ Error ZIPPacker::close() {
 }
 
 Error ZIPPacker::start_file(String p_path) {
-	ERR_FAIL_COND_V_MSG(zf != NULL, FAILED, "ZIPPacker is already in use.");
 	ERR_FAIL_COND_V_MSG(fa.is_null(), FAILED, "ZIPPacker must be opened before use.");
 
 	zip_fileinfo zipfi;