[batch] Ignore empty command strings when constructing shell scripts

Using job.command('') (or more likely something more complex
that sometimes evaluates to an empty string) led to shell snippets
containing `... { } ...`, which is a shell syntax error. Prevent
this by not adding such strings to self._command and warning.

hail/python/hailtop/batch/job.py

@@ -859,6 +859,11 @@ def command(self, command: str) -> 'BashJob':
  Same job object with command appended.
  """
 
+ if command.strip() == '':
+ job_name = f'job {self.name!r}' if self.name else 'unnamed job'
+ warnings.warn(f'Ignoring empty command specified for {job_name}.')
+ return self
+
  command = self._interpolate_command(command)
  self._command.append(command)
  return self