Skip empty lines from tox deps

An empty line happens when there are not deps:

    Handling  from tox --print-deps-only: <toxname>
    WARNING: Skipping invalid requirement:
        Parse error at "''": Expected W:(abcd...)

pyproject_buildrequires.py

@@ -235,7 +235,7 @@ def parse_tox_requires_lines(lines):
                 f'WARNING: Skipping dependency line: {line}\n'
                 + f'    tox deps options other than -r are not supported (yet).',
             )
-        else:
+        elif line:
             packages.append(line)
     return packages