check for falsy mappings

[ahvigil]

Motivation

Address the failure in #726 by checking for falsy Map values in the same way the branch above does for Arrays.

Approach

Just mirroring the handling of empty Arrays!

Checklist

• Add appropriate test(s) to the automatic suite
• Use make pretty to reformat the code with black
• Use make check to statically check the code using Pyre and Pylint
• Send PR from a dedicated branch without unrelated edits
• Ensure compatibility with this project's MIT license

[ahvigil]

I checked this against the demo workflow in the linked issue to confirm this addresses the problem:

$ miniwdl run --no-cache demo.wdl
...
2025-02-21 15:02:51.319 wdl.w:demo done
{
  "dir": ".../20250221_150248_demo",
  "outputs": {
    "demo.echo.output_map": {}
  }
}