Issue #13234 - empty query entries should be retained for Fields and Servlet Parameters

[joakime]

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Fix regression in 12.0.21, where empty query parameters should still produce a Field or Servlet Parameter.

That means ..

1. ?= is a single parameter with key name of empty string (""), with a single value of empty string ("").
2. ?=&= is a single parameter with key name of empty string (""), with a two values of empty string ("").
3. ?& has no parameters.
4. ?&& has no parameters.

Closes #13234