Fix filter_var with callback and explicit REQUIRE_SCALAR

For some reason, FILTER_CALLBACK disables the FILTER_REQUIRE_SCALAR flag that is
normally set by default. While surprising, this

Fix filter_var with callback and explicit REQUIRE_SCALAR

For some reason, FILTER_CALLBACK disables the FILTER_REQUIRE_SCALAR flag that is
normally set by default. While surprising, this is not something we can change.

However, even specifying FILTER_REQUIRE_SCALAR explicitly does not corrently set
this flag. This is because FILTER_CALLBACK zeroes the flags after they have been
populated from the parameters.

We reverse the checks to make explicitly specifying the flag behave as expected.

Closes GH-12203

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