show create generates invalid default values for columns

[jeroenpf]

The default column values in create table statements after running show create table contain invalid default values that do not correspond to the original tables.

A non-exhaustive list of examples:

• timestamp/datetime NOT NULL without a default value appears with the default value 0 in the resulting create statement.
• decimal(x,y) NOT NULL without a default value appears with a default value 0 in the resulting create statement.
• bigint(x) NOT NULL 0 appears without a default value in the resulting create statement

This issue seems to happen when we run create statements and force NOT NULL columns to always have a default value. This is likely necessary for compatibility with WordPress queries that don't expect strict checks. We could consider storing the original default value (if any) in the _mysql_data_types_cache table and use that as the source of truth when we run show create queries.