tests: Replace PHPUnit's loose assertEquals(null) with assertNull()

tests: Replace PHPUnit's loose assertEquals(null) with assertNull()

assertEquals( null, … ) still succeeds when the actual value is 0, false,
an empty string, even an empty array. All these should be reported as a
failure, I would argue.

Note this patch previously also touched assertSame( null ). I reverted
these. The only benefit would have been consistency within this codebase,
but there is no strict reason to prefer one over the other. assertNull()
and assertSame( null ) are functionally identical.

Change-Id: I92102e833a8bc6af90b9516826abf111e2b79aac