codeaucafe/NOISSUE: remove redundant beginTableCalled variable

[codeaucafe]

As noted by @elianddb in my PR adding --filter option to dolt diff I was using a redundant bool variable that was already available on the mockDiffWriter.

This is a small trivial PR to remove the standalone beginTableCalled variable in TestLazyRowWriter_NoRowsWritten test function. The test now uses mockDW.beginTableCalled directly, which maintains consistency with other test functions (TestLazyRowWriter_RowsWritten and TestLazyRowWriter_CombinedRowsWritten) that already follow this pattern.

Note: please feel free to cancel/close this PR if it's not necessary, to small, or something else. I put up the PR because it was fast and the type of thing that will keep bothering me 🤣