apidocs: Fix invalid API page bug.
apidocs: Fix invalid API page bug.

The current logic to get API pages' title using
OperationID should be used when the first line
of the file explicitly mentions so.

In cases where the files didn't begin with `#` but also
didn't need to get title from OpenAPI summary,
this logic fails and causes Server error.
This particularly happens when the article is invalid,
and the `missing.md` file doesn't need title to be
generated, but doesn't start with `#` either.

This commit fixes the logic of using the generated title and covers the bug.

eeshangarg/zulip,rht/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,rht/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip