Fix the schema fetching during sink storage assignment when using MongoDB

[bobbai00]

This PR fixes the issue that, when MongoDB is used as the result storage, the execution keeps failing.

The root cause is:

• when using MongoDB as the result storage, the schema has to be extracted from the physical op
• the code implementation uses the physical ops in subPlan to extract the schema out, which is incorrect as subPlan's physical ops are not propagated for output schemas

How the fix is done is: using physicalPlan variable, instead of subPlan.