pivot-table: Change the default format to omit empty rows and columns.

pivot-table: Change the default format to omit empty rows and columns.

By making this the default, it doesn't become necessary to make a lot of
procedures that yield output change the format to produce reasonable
output.  This means that the user's settings for the format won't get
overridden.

This changes a lot of output that had empty columns, in particular
anything that used DISPLAY DICTIONARY with a dictionary that had no
variable labels or missing values.  This commit also updates the expected
output to match.

An unexpected side effect was that some tables became empty entirely, for
example LIST with no rows.  This seemed undesirable, so I changed
pivot_table_enumerate_axis() so if an axis was entirely empty due to
omit_empty, then omit_empty was suppressed for that axis.