Merge branch 'jk/submodule-c-credential'

"git -c credential.<var>=<value> submodule" can now be used to
propagate configuration variables related to credential helper
down to the submodules.

* jk/submodule-c-credential:
  git_config_push_parameter: handle empty GIT_CONFIG_PARAMETERS
  git: submodule honor -c credential.* from command line
  quote: implement sq_quotef()
  submodule: fix segmentation fault in submodule--helper clone
  submodule: fix submodule--helper clone usage
  submodule: check argc count for git submodule--helper clone
  submodule: don't pass empty string arguments to submodule--helper clone

config.c

@@ -162,7 +162,7 @@ void git_config_push_parameter(const char *text)
 {
 	struct strbuf env = STRBUF_INIT;
 	const char *old = getenv(CONFIG_DATA_ENVIRONMENT);
-	if (old) {
+	if (old && *old) {
 		strbuf_addstr(&env, old);
 		strbuf_addch(&env, ' ');
 	}