[HttpFoundation] Fixed bug in key searching for NamespacedAttributeBag

Fixed a bug in NamespacedAttributeBag causing a result to be falsely found when
the last key of the attribute matched the last of the queried name regardless of
if the key did not exist in the search.
Added Tests to demonstrate the issue and resolved by setting keys to null when
iterating through query and returning proper responses in the case that the
given array does in fact not exist.

* Updated Syntax of null checks
* Fixing missing else case for if statement in write context
merged branch MaxVandervelde/fix/namespaced-parameter-issue (PR #7586)

This PR was merged into the 2.1 branch.

Discussion
----------

[HttpFoundation] Fixed bug in key searching for NamespacedAttributeBag

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7564
| License       | MIT
| Doc PR        | N/A

Fixed a bug in NamespacedAttributeBag causing a result to be falsely found when
the last key of the attribute matched the last of the queried name regardless of
if the key did not exist in the search.
Added Tests to demonstrate the issue and resolved by setting keys to null when
iterating through query and returning proper responses in the case that the
given array does in fact not exist.

Commits
-------

0f0c29c [HttpFoundation] Fixed bug in key searching for NamespacedAttributeBag