Exclude Object when checking for DbContext in query filters

[roji]

Fixes #18759

Ports #18761 to 3.1.

[Pilchie]

@ajcvickers / @roji can you fill out the ask mode template with customer scenario, workarounds, etc?

[roji]

Description

Since 3.0, null constants in query filters get incorrectly parameterized and cause query translation failure unless compared against a type that defines an equality operator (==).

Customer Impact

Query failure when defining a query filter comparing either navigation properties to null, or non-navigation reference properties such as byte arrays, spatial types which don't define an equality operator. Filters that compare to null are common.

How found

Customer-reported (#18759)

Test coverage

We had a test for an extremely close scenario, but unfortunately our test entity types define an equality operator (==), so the bug wasn't triggered.

Regression?

Yes, from 2.2.

Risk

Risk is very small - the fix is adding a single check to exclude Object in a visitor (to prevent the null constant from mistakenly being identified as the context type).

[leecow]

Approved for GA. Please hold for branch to open.

[roji]

@smitpatel addressed in #18918