fix(postgrest): build a valid Prefer header in select() when none is set

[spydon]

Stacked on #1432.

What

This came out of investigating avoid-nullable-interpolation (PR scope: only the meaningful sites — most of that rule's 80 hits are cosmetic toString/log interpolation, so the rule stays disabled). One site turned out to be a real bug:

PostgrestTransformBuilder.select() builds the Prefer header like:

if (newHeaders['Prefer'] != null) {
  newHeaders['Prefer'] = '${newHeaders['Prefer']},';
}
newHeaders['Prefer'] = '${newHeaders['Prefer']}return=representation';

The second line runs unconditionally. When no Prefer header is set yet, newHeaders['Prefer'] is null and the interpolation produces the malformed header nullreturn=representation.

insert()/update()/delete() set Prefer = '' first, so their .select() is unaffected — but rpc(...).select() does not set Prefer, so it was sending the malformed header.

Fix: default the missing value to an empty string (${newHeaders['Prefer'] ?? ''}). No change when Prefer is already set.

Also

• Regression test in the custom-http-client group: rpc('empty-succ').select().head() now sends Prefer: return=representation (fails before this fix with nullreturn=representation).
• Dropped two redundant this. qualifiers in postgrest.dart left over from an earlier url -> requestUrl rename.

Verification

• New test passes with the fix, fails without it (confirmed by temporarily reverting).
• dart analyze clean (incl. infos); dcm analyze packages -> no issues found.
• postgrest mock suites pass.

Note: avoid-nullable-interpolation itself remains disabled (the toString/log majority would still violate it).