fixes # 145 "empty files causing testmon to crash"

#145 Fix empty files causing testmon to crash

We were creating Module based on code variable value. When file is empty code was empty string and we didnt create Module instance, instead we returned None. This was causing crash in make_nodedata because file exists but module instance was None.

Not we create Module instance based on checksum value. Checksum is defined even if file is empty.