Fix nullability analysis in SRHTTPConnectMessage.

SocketRocket/Internal/Utilities/SRHTTPConnectMessage.m

@@ -42,12 +42,14 @@ CFHTTPMessageRef SRHTTPConnectMessageCreate(NSURLRequest *request,
     }
 
     // Apply cookies if any have been provided
-    NSDictionary<NSString *, NSString *> *messageCookies = [NSHTTPCookie requestHeaderFieldsWithCookies:cookies];
-    [messageCookies enumerateKeysAndObjectsUsingBlock:^(NSString * _Nonnull key, NSString * _Nonnull obj, BOOL * _Nonnull stop) {
-        if (key.length && obj.length) {
-            CFHTTPMessageSetHeaderFieldValue(message, (__bridge CFStringRef)key, (__bridge CFStringRef)obj);
-        }
-    }];
+    if (cookies) {
+        NSDictionary<NSString *, NSString *> *messageCookies = [NSHTTPCookie requestHeaderFieldsWithCookies:cookies];
+        [messageCookies enumerateKeysAndObjectsUsingBlock:^(NSString * _Nonnull key, NSString * _Nonnull obj, BOOL * _Nonnull stop) {
+            if (key.length && obj.length) {
+                CFHTTPMessageSetHeaderFieldValue(message, (__bridge CFStringRef)key, (__bridge CFStringRef)obj);
+            }
+        }];
+    }
 
     // set header for http basic auth
     NSString *basicAuthorizationString = SRBasicAuthorizationHeaderFromURL(url);