[PHP] Accept 0 as value for query parameters

Created by: thomasphansen

Fixes #13635 (closed), by correcting a regression added by #11225 and extending #12120 adjustments.

ObjectSerializer::toQueryValue() filters out parameters whose values are "empty" (using the empty() method from php), but it disregards the value's type when doing so. A single exception is added for the false value, if the OpenApi type is boolean.

This PR add checks for numeric fields, since the value "0" (both integer and float) should not be filtered out. New tests are added, covering 100% of the added lines.

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