builtin/push: call set_refspecs after validating remote

When an end-user runs "git push" with an empty string for the remote
repository name, e.g.

    $ git push '' main

"git push" fails with a BUG(). Even though this is a nonsense request
that we want to fail, we shouldn't hit a BUG().  Instead we want to give
a sensible error message, e.g., 'bad repository'".

This is because since 9badf97c42 (remote: allow resetting url list,
2024-06-14), we reset the remote URL if the provided URL is empty. When
a user of 'remotes_remote_get' tries to fetch a remote with an empty
repo name, the function initializes the remote via 'make_remote'. But
the remote is still not a valid remote, since the URL is empty, so it
tries to add the URL alias using 'add_url_alias'. This in-turn will call
'add_url', but since the URL is empty we call 'strvec_clear' on the
`remote->url`. Back in 'remotes_remote_get', we again check if the
remote is valid, which fails, so we return 'NULL' for the 'struct
remote *' value.

The 'builtin/push.c' code, calls 'set_refspecs' before validating the
remote. This worked with empty repo names earlier since we would get a
remote, albeit with an empty URL. With the new changes, we get a 'NULL'
remote value, this causes the check for remote to fail and raises the
BUG in 'set_refspecs'.

Do a simple fix by doing remote validation first. Also add a test to
validate the bug fix. With this, we can also now directly pass remote to
'set_refspecs' instead of it trying to lazily obtain it.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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