Fix optional parameters in strings generation

Sources/ResgenSwift/Generate/Runnable/StringsConfiguration+Runnable.swift

@@ -38,8 +38,8 @@ extension StringsConfiguration: Runnable {
             ("--extension-suffix", extensionSuffix),
             ("--visibility", visibility),
             ("--asset-bundle", assetBundle),
-            ("--xc-strings", staticMembers?.description),
+            ("--xc-strings", xcStrings?.description),
-            ("--static-members", xcStrings?.description)
+            ("--static-members", staticMembers?.description)
         ].forEach { argumentName, argumentValue in
             if let argumentValue {
                 args += [