BUG: blockMesh mergePatchPairs fails with edge-shared points (fixes #2589)

- remedy by performing the attach() action sequentially (as per
  stitchMesh changes). This ensures that the current point addressing
  is always used and avoids references to the already-merged points
  (which is what causes the failure).

ENH: improve handling of empty patch removal

- only remove empty *merged* patches, but leave any other empty
  patches untouched since they may intentional placeholders for other
  parts of a workflow.

- remove any empty point/face zones created for patch merging