[clang][analyzer] Add report of NULL stream to StreamChecker.

The report of NULL stream was removed in commit 570bf972.
The old reason is not actual any more because the checker dependencies are changed.
It is not good to eliminate a failure state (where the stream is NULL) without
generating a bug report because other checkers are not able to find it later.
The checker did this with the NULL stream pointer, and because this checker
runs now before other checkers that can detect NULL pointers, the null pointer
bug was not found at all.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D152169