Relax the warning that emanates from not returning a dict from `execute`

[davemfish]

InVEST model execute functions return a dictionary representing the file registry. This is used in metadata generation and in reports. But if a plugin does not return anything from execute, we should tolerate this. Right now it results in a warning, which includes a caught exception,

2026-04-20 14:02:52  natcap.invest.spec WARNING  Something went wrong while generating metadata
Traceback (most recent call last):
  File "/Users/<>/Library/Application Support/invest-workbench/micromamba_envs/plugin_1776689697943/lib/python3.13/site-packages/natcap/invest/spec.py", line 2260, in execute
    self.generate_metadata_for_outputs(
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        registry, preprocessed_args)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/<>/Library/Application Support/invest-workbench/micromamba_envs/plugin_1776689697943/lib/python3.13/site-packages/natcap/invest/spec.py", line 2115, in generate_metadata_for_outputs
    for key, value in file_registry.items():
                      ^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'items'

This is a bit heavy-handed. It's okay for plugins to return None from execute so we shouldn't issue a message that makes people think something went wrong.

Benefits

The bar to getting a plugin working will be lowered because developers will not necessarily need to create an output specification or use a file_registry object before getting a functional plugin.

Mitigation of side effects

Fewer developers may opt in to using these features (file_registry and metadata) which is not a good thing. We can provide documentation and examples of plugins that do use these features in order to encourage these practices.