Replace assertTrue with assertIsNotNone for check an object

assertTrue(*) means to check whether a boolean value/expression
is true or not. For checking an object is not None, we should use
assertIsNotNone(*), which gives more explicit expression.

Change-Id: Ia4d0dcec81e97fe4774bff4e41a017f00b899ef3
Closes-Bug: #1297156
Merge "Replace assertTrue with assertIsNotNone for check an object"