Fix empty lines in dimacs

[jjcmoon]

Encountered a bug where .cnf files couldn't be loaded if they contain empty lines (see below).
Fixed by moving the empty line check up.

  File ".../quimb/tensor/tensor_builder.py", line 2607, in cnf_file_parse
    if args[0] == "p":
IndexError: list index out of range

[pep8speaks]

Hello @jjcmoon! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file quimb/tensor/tensor_builder.py:

Line 2761:1: W293 blank line contains whitespace

[jcmgray]

LGTM, thanks!