Just: drop a check the pattern above already makes

An assignment only reaches this loop by matching `ENV_RE`, which requires a
name before the `=`, so the key can never come back empty and the branch
could not run.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

misc/just/codeql_test_run.py

@@ -130,9 +130,6 @@ def main():
 
     for env_var in args.env:
         key, _, value = env_var.partition("=")
-        if not key:
-            error(f"Invalid environment variable assignment: {env_var}")
-            return 1
         os.environ[key] = value
 
     # Resolve codeql executable