Blank brush strokes never have an oppurtunity to initialize
a sculpt undo step, which results in blank global undo steps.
These confuse DynTopo's internal BMLog undo stack.
Note: I tried having the stroke operator return OPERATOR_CANCELLED,
but this didn't prevent the undo push (which is done automatically
due to the presence of OPTYPE_UNDO in the operator flags).
I might investigate removing the flag, but that might cause other
problems.
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