local_resource will not accept explicit None for readiness_probe value

[milas]

Expected Behavior

local_resource('foo', serve_cmd='./myserver', readiness_probe=None) should behave the same as if the kwarg was omitted

Current Behavior

local_resource('foo', serve_cmd='./myserver', readiness_probe=None) results in an error:

Error: local_resource: for parameter "readiness_probe": got NoneType, want struct

This makes the ergonomics awkward when dynamically generating a resource that may or may not have a probe.