Unpublishing from external stores null pointer exception fixed.

[shnrndk]

Purpose

This PR fixes the Internal Server Error while Trying to Remove All Published External API Developer Portals from an API issue.
Related to https://github.com/wso2-enterprise/wso2-apim-internal/issues/2465

Approach

NullPointerException is happening in the org/wso2/carbon/apimgt/rest/api/publisher/v1/impl/ApisApiServiceImpl.java:2182.

• ExternalStoreId parameter is missing in the request.
• Since ExternalStoreId is null,

  List<String> externalStoreIdList = Arrays.asList(externalStoreIds.split("\\s*,\\s*"));
  

  splitting occurs the null pointer exception.
• Added an if condition to check whether the ExternalStoreId is null, and if it is null then it will assign a new empty arraylist.