fix(usercontext): Add null check for user properties

This adds additional checks for user parameters

Fixes #211

Author: HazAT

ios/RNSentry.m

@@ -391,9 +391,15 @@ - (SentryUser *_Nullable)createUser:(NSDictionary *_Nonnull)user {
         userId = [NSString stringWithFormat:@"%@", user[@"id"]];
     }
     SentryUser *sentryUser = [[SentryUser alloc] init];
-    sentryUser.userId = userId;
-    sentryUser.email = [NSString stringWithFormat:@"%@", user[@"email"]];
-    sentryUser.username = [NSString stringWithFormat:@"%@", user[@"username"]];
+    if (nil != userId) {
+        sentryUser.userId = userId;
+    }
+    if (nil != user[@"email"]) {
+        sentryUser.email = [NSString stringWithFormat:@"%@", user[@"email"]];
+    }
+    if (nil != user[@"username"]) {
+        sentryUser.username = [NSString stringWithFormat:@"%@", user[@"username"]];
+    }
     sentryUser.extra = [RCTConvert NSDictionary:user[@"extra"]];
     return sentryUser;
 }