Srclib tries to run graph op when it's null and errors

[xizhao]

command failed: src tool --methods program "sourcegraph.com/sourcegraph/srclib-javascript" "" < .srclib-cache/713cda968b801550271ba099d1cbba63b7edb01b/async/CommonJSPackage.unit.json | src internal normalize-graph-data --unit-type "CommonJSPackage" --dir . 1> .srclib-cache/713cda968b801550271ba099d1cbba63b7edb01b/async/CommonJSPackage.graph.json (exit status 1) 

I intentionally omit the tc graph ops in Srclibtoolchain to speed up builds
Srclib still tries to run it, causing it to error as above
GraphUnitRule should be excluded or blank if the Op is ""
Either that or the tc on import should omit the graph step if it is null

[xizhao]

To clarify it's Tools.[i].Subcmd that's the main issue. Another way is to suppress this error and allow builds to clear even without a graph step:
Error: Command failed: rule maker graph: no tool satisfies op "graph" for source unit type "CommonJSPackage"