[utils/cli] Accept empty string as a falsey value in strtobool

Some UE asset manifests seem to have `"OwnershipToken": ""` instead
of something `strtobool` could understand. When trying to import these
through `egl-sync` an exception is being raised, which blocks importing
other titles too.

The other use of `strtobool` is in `LGDRY_NO_WINE` environment variable,
which also causes an exception if set as `LGDRY_NO_WINE=`

legendary/utils/cli.py

@@ -84,7 +84,7 @@ def strtobool(val):
     val = val.lower()
     if val in ('y', 'yes', 't', 'true', 'on', '1'):
         return 1
-    elif val in ('n', 'no', 'f', 'false', 'off', '0'):
+    elif val in ('n', 'no', 'f', 'false', 'off', '0', ''):
         return 0
     else:
         raise ValueError("invalid truth value %r" % (val,))