Dates: Error on construction/parsing with empty strings (#47117)
When attempting to construct a `DateTime`, `Date` or `Time` from an
`AbstractString`, throw an `ArgumentError` if the string is empty.
Likewise, error when `parse`ing an empty string as one of these types,
and return `nothing` from `tryparse`.
This behavior differs from previously. Before, `Date` and `Time` would
return default values of `Date(1)` and `Time(0)`, respectively, while
`DateTime` would error without a `format` argument. With a `format`
argument, it would return `DateTime(1)`. However, this appears not to
have been explicitly intended, but rather a consequence of the way
parsing was implemented; no tests for empty string parsing existed.
This addresses #28090 and #43883; see discussion therein.
Summary of changes:
- Check for empty string in `Base.parse(::DateTime)` and throw if so.
- Change documentation to mention this.
- Add a compat notice to the docs contrasting the old behavior.
