Conditional request handling treats the Unix epoch as a missing date

[canblmz]

parse_http_date_safe() returns 0 for Thu, 01 Jan 1970 00:00:00 GMT, and the conditional request code checks these timestamps for truthiness, so the epoch is handled as if the date were missing:

• get_conditional_response() ignores If-Unmodified-Since: Thu, 01 Jan 1970 00:00:00 GMT, so a request that should get 412 (for any resource modified after 1970) goes through.
• _if_unmodified_since_passes() and _if_modified_since_passes() treat a resource whose Last-Modified is the epoch as having no modification date: If-Unmodified-Since always gives 412, and If-Modified-Since never gives 304.
• ConditionalGetMiddleware passes an epoch Last-Modified on as missing, and skips conditional handling altogether when the response has no ETag.
• @condition / @last_modified don't add the Last-Modified header when last_modified_func returns the epoch.

An epoch Last-Modified can come up for files whose mtime was normalized to 0, e.g. by a reproducible build.

Comparing with is None / is not None instead fixes all four. I have a patch with tests ready.

This touches the same helper as #37252 (​PR 21737), where not last_modified would also treat the epoch as missing. The two fixes are compatible, and I'm happy to rebase on whichever lands first.