T 1164100: Add check for nil request URL

[ruchiupgade-mb]

• I've read, understood, and done my best to follow the CONTRIBUTING guidelines.

This pull request includes (pick all that apply):

• Bugfixes
• New features
• Breaking changes
• Documentation updates
• Unit tests
• Other

Summary

In URLSessionClient if the modifiedRequest URL is nil, the data task was not getting initialized. Because of this The callback is not fired.

In this fix we have added a check for nil URL. If the request URL is nil we are initiating a callback with missingURL error.

Implementation

Test Plan

[plflanagan]

Just one quick change request. Let me know when made and I can quickly re-review.