ReflectionTestUtils.setField() implicitly calls toString() on the target
object when arguments for a call to Assert.notNull() are built. This can
have undesirable side effects, for example if the toString() invocation
results in a thrown exception or access to an external system (e.g., a
database).
This commit addresses this issue by inlining the Assert.notNull() code,
thereby avoiding accidental invocation of toString() on a non-null
target.
Issue: SPR-9571
df961a938e