Merge two table on two columns with OR operator

[Laila92]

Looking to merge two table on two columns A and B. However, I want to do the following: 

Try to match on column A and ONLY IF column A is empty, try to find a match in column B. 

What I have now:

#"Merged Queries2" = Table.NestedJoin(#"Merged Queries", {"A","B"}, Activities, {"A","B"}, "Activities2", JoinKind.FullOuter),

Looking for something like (but including the is null for column A condition):

#"Merged Queries2" = Table.NestedJoin(#"Merged Queries", {"A"||"B"}, Activities, {"A"||"B"}, "Activities2", JoinKind.FullOuter),

Is this possible in PowerBI?

[Crow2525]

I think your answer might be in your question.

Give the Try and Catch a crack?

1. Goodly - Youtube - Try and Catch in Power Query | Smarter way to work with errors in Power Query 

2. Error handling - Power Query | Microsoft Docs

Disclaimer: I'm a novice and learning