Don't store all-null dimension columns

When a Druid segment is created, it could be checked that a dictionary-encoded dimension column contains all null values, and just not written to the segment if so.

Probably the same couldn't be done for all-zero numeric columns, because if they are queried as dimensions they should appear as dimensions with all "0" values, not "null" values.

No files, tests, or entry points are named in the issue. Start by tracing Druid segment creation and dictionary-encoded dimension-column writing, then check how omitted all-null columns behave when queried. Done means all-null dimension columns are not written while their query semantics remain correct.

Written by the indexing model from the issue text.