Make SpecialPreferencesTest::testBug41337 non-risky

PHPUnit treats tests that make no assertions as "risky" because they make it
difficult to distinguish successful test runs from programmer error. To make
SpecialPreferencesTest::testBug41337 non-risky, assert that the execute method
returns null.

Change-Id: I4c199d2b81037e6ca2b2862418c4f8a8352c777e
Merge "Make SpecialPreferencesTest::testBug41337 non-risky"