tests: Prefer PHPUnit's assertSame() when comparing empty strings

tests: Prefer PHPUnit's assertSame() when comparing empty strings

assertSame() is guaranteed to never do any magic type conversion.
This can be critical when accidentially comparing empty strings (a
value PHP considers to be "falsy") to false, 0, 0.0, null, and such.

Change-Id: I2e2685c5992cae252f629a68ffe1a049f2e5ed1b