Treat an omitted executeTool() input as an empty object

[MiguelsPizza]

@domfarolino @beaufortfrancois I ran into this while writing the polyfill. Not sure what the intended behavior is, but I prefer the fallback:

document.modelContext.executeTool(tool); // this should just work IMO

clanker with the details below:

Right now the spec says calling executeTool(tool) with no input throws a TypeError, but Chrome and WPT run the tool with {}.

The spec used to default to {} too. #251 changed the input's type to any and first kept the default as a step (45afd88), but its last commit removed that step (b755b64) when Chromium moved the default into its C++ code. This PR puts the step back.

One wrinkle: the CL review wanted an explicit undefined to throw, but Web IDL treats it the same as a missing argument, so executeTool(tool, undefined) gets {} too. Chrome already does this.

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[beaufortfrancois]

Sorry for missing this bit in https://github.com/webmachinelearning/webmcp/pull/251/changes.

@domfarolino WDYT?

[domfarolino]

Thanks, yeah I think this is a bug. The behavior I really wanted in https://crrev.com/c/8250826/comment/43a901d5_d0bb54d8/ was:

• Missing input arguments would be treated just like executeTool(tool, {}).
• Explicitly passing undefined, as in executeTool(tool, undefined), would throw an exception.

But Web IDL intentionally treats missing undefined and explicit undefined for optional arguments (with no default) as "missing", so I don't think we can achieve my desired distinction without introducing an overload:

executeTool(tool, options)
executeTool(tool, object inputArguments, options)

But that's not very clean, and I'm not sure it's worth fighting Web IDL's defaults here. So I'm happy with the direction of this PR: converge missing-undefined and explicit-undefined. See the two notes below.

[domfarolino]

Alright I've pushed up a few changes to address the above. Would like one more person on this thread to give an LGTM and I think we can merge.

[MiguelsPizza]

LGTM! Thanks folks

[beaufortfrancois]

LGTM with nit

[anssiko]

I dropped this from the agenda since we're aligned and can land this fix soon. Thanks for the PR!