refactor: use optional chaining for sourceMapCallback

[agilgur5]

Summary

Tiny one-liner change to use optional chaining

• following other optional chaining simplifications like refactor(cache): simplify condition w/ optional chaining #356, refactor: invert some conditionals for better readability #335

Misc

Is sourceMapCallback actually used by anyone? It's not listed in the docs so I didn't know it was a thing until I saw it in the code

[agilgur5]

Is sourceMapCallback actually used by anyone? It's not listed in the docs so I didn't know it was a thing until I saw it in the code

Seems like it originates from #69 and that is the only reference to it in the entire issue/PR landscape, so that contributor may be the only person to use it 😅

The use-case may also no longer be necessary with newer Istanbul versions (and nyc existing) based on some of the upstream comments linked, or with better source map handling in Rollup, but really hard to tell with such a complicated use-case without a simple repro

[ezolenko]

The problem with having many options is that once they are in, they tend to stay in, unless you have lots of telemetry and actively trim unused ones...

[agilgur5]

Yea one of the reasons to be careful of introducing new features is the long-term maintenance cost 😕

Telemetry not being a thing in OSS also makes these things super hard to track too; there's a space for it for sure, albeit controversial