alias: treat empty subsection [alias ""] as plain [alias]

alias: treat empty subsection [alias ""] as plain [alias]

When git-config stores a key of the form alias..name, it records
it under an empty subsection ([alias ""]). The new subsection-aware
alias lookup would see a non-NULL but zero-length subsection and
fall into the subsection code path, where it required a "command"
key and thus silently ignored the entry.

Normalize an empty subsection to NULL before any further processing
so that entries stored this way continue to work as plain
case-insensitive aliases, matching the pre-subsection behaviour.

Users who relied on alias..name to create an alias literally named
".name" may want to migrate to subsection syntax, which looks less confusing:

    [alias ".name"]
        command = <value>

Add tests covering both the empty-subsection compatibility case and
the leading-dot alias via the new syntax.

Signed-off-by: Jonatan Holmgren <jonatan@jontes.page>
Signed-off-by: Junio C Hamano <gitster@pobox.com>