Allow symbolic files of size 0

[moralismercatus]

@likebreath @UnseeingEye

Currently, when a file of size 0 is listed as a symbolic input file, CRETE raises an assertion. Presumably, the reasoning was that the user likely made a mistake.

However, there is nothing in the file system against files of size 0, and it would make CRETE consistent with the FS.

In our particular case, this restriction caused problems with our infrastructure which was automatically generating files.

As a mitigation, I changed the assertion to treat symbolic files of size 0 to concrete:

https://github.com/moralismercatus/crete-dev/blob/exciting/lib/include/crete/harness_config.h#L617