row_build_index_entry_low(): Clean up the code, removing
redundant variables and conditions. For instantly dropped columns,
generate a dummy value that is NULL, the empty string, or a
fixed length of NUL bytes, depending on the type of the dropped column.

FIXME: For some reason, during the test gcol.innodb_virtual_debug_purge
we are returning NULL for indexes on virtual columns, meaning that
purge on the indexes is being skipped. Does this lead to missed purge,
or is it merely causing unnecessary memory allocation? Why are we not
evaluating the virtual column values before calling this function?
(And why not write all index keys (virtual columns or spatial index MBR)
directly to the undo log records?)