OAuth2: userinfo request crashes with MissingSchema when OAUTH2_USERINFO_ENDPOINT is present but None

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Describe the bug

web/pgadmin/authenticate/oauth2.py:747 guards the userinfo request like this:

if 'OAUTH2_USERINFO_ENDPOINT' not in self.oauth2_config[...]:
    ...
    'OAUTH2_USERINFO_ENDPOINT not configured for ...'
...
client.get(self.oauth2_config[self.oauth2_current_client]['OAUTH2_USERINFO_ENDPOINT'])

This tests key presence, not truthiness. But web/config.py:864 ships the OAUTH2_CONFIG template entry with:

'OAUTH2_USERINFO_ENDPOINT': None,

So a config copied from the shipped template (as documented) has the key present with a None value. The not in check passes (key exists), so the code proceeds to call client.get(None), which raises requests.exceptions.MissingSchema: Invalid URL 'None' instead of either working correctly (for an OIDC provider that supplies claims via ID token / discovery, where this endpoint genuinely isn't needed) or failing with the intended "not configured" message.

To Reproduce

1. Configure OAUTH2_CONFIG for an OIDC provider using OAUTH2_SERVER_METADATA_URL, leaving OAUTH2_USERINFO_ENDPOINT at its template default of None (rather than deleting the key).
2. Log in via that provider.
3. The userinfo fetch raises MissingSchema: Invalid URL 'None' instead of skipping the call or falling back to discovery-resolved metadata.

Expected behavior

The check should test truthiness (if not self.oauth2_config[...].get('OAUTH2_USERINFO_ENDPOINT')) rather than key presence, so an explicit None is treated the same as an absent key.

Found while re-verifying #8804 (OIDC discovery) — the discovery mechanism itself works correctly, but this adjacent code path in the same login flow has a real crash for configs that follow the shipped template.