fix and add tests

Author: lobsterkatie

packages/core/src/baseclient.ts

@@ -641,7 +641,7 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
       .then(prepared => {
         if (prepared === null) {
           this.recordDroppedEvent('event_processor', event.type || 'error');
-          throw new SentryError('An event processor returned null, will not send event.', 'log');
+          throw new SentryError('An event processor returned `null`, will not send event.', 'log');
         }
 
         const isInternalException = hint.data && (hint.data as { __sentry__: boolean }).__sentry__ === true;