[analyzer] Properly handle the ternary operator in trackNullOrUndefValue

1) Look for the node where the condition expression is live when checking if
it is constrained to true or false.

2) Fix a bug in ProgramState::isNull, which was masking the problem. When
the expression is not a symbol (,which is the case when it is Unknown) return
unconstrained value, instead of value constrained to “false”!
(Thankfully other callers of isNull have not been effected by the bug.)

llvm-svn: 178684