Add null pointer check and testcase for snapsync tasks

[pullurib]

PR description

Added a null pointer check where a null pointer exception was thrown as described in the issue
Added a testcase to verify the case is handled gracefully

taskElement shouldn't be null ideally as it's only added to trie after validating proofs and we get to this point after checking if data is available and isValidProof is true.

• A race condition between calling addResponse(requestDataStep) and getChildRequests(persistDataStep) seems unlikely as they are sequential in pipeline and synchronized by setting isValidProof after adding taskElement and checked before accessing taskElement.
• Clear() on the request also sets isValidProof to false before removing the element from Trie.

Fixed Issue(s)

#6113

[lu-pinto]

LGTM