Add null check for ApiKeyPair in getUserByApiKey

The function getUserByApiKey was not checking for null pointers. I think this is safe from the perspective of security, but this is a smell. And it can be easily addressed with the same solution used in line 3160.

Author: daviftorres

server/src/main/java/com/cloud/user/AccountManagerImpl.java

@@ -3828,6 +3828,9 @@ public void buildACLViewSearchCriteria(SearchCriteria<? extends ControlledViewEn
     @Override
     public UserAccount getUserByApiKey(String apiKey) {
         ApiKeyPairVO keyPair = apiKeyPairDao.findByApiKey(apiKey);
+        if (keyPair == null) {
+            return null;
+        }
         return userAccountDao.findById(keyPair.getUserId());
     }