esv: initialise cFlag to null

Before this change a user could pass an empty string as the argument
to -C and it would go undetected.

esv.d

@@ -58,6 +58,8 @@ main(string[] args)
 
 	sharedInit(args);
 
+	cFlag = null;
+
 	/* Parse command-line options */
 	try {
 		import std.getopt : config;
@@ -106,7 +108,7 @@ config:
 	configPath = environment.get(cf.configEnv, cf.configPath)
 		.expandTilde();
 	try {
-		if (cFlag != "") { /* if -c was given */
+		if (cFlag) {
 			configPath = cFlag.expandTilde();
 		} else {
 			if (!configPath.exists()) {