fix: if preprocessor is async function and doesn't return a content t…

[anthony-redFox]

…hen await donePromise

[anthony-redFox]

@johnjbarton please take a look

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[karmarunnerbot]

✅ Build karma 33 completed (commit 3f9ca408af by @anthony-redFox)

[karmarunnerbot]

✅ Build karma 32 completed (commit 3f9ca408af by @anthony-redFox)

[johnjbarton]

Please add a test for the normal async path where done() is not called.

[anthony-redFox]

@johnjbarton Thanks for review. Add new unit tests and comments

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✅ Build karma 2432 completed (commit 23033eaa42 by @anthony-redFox)

[karmarunnerbot]

✅ Build karma 33 completed (commit 23033eaa42 by @anthony-redFox)

[karmarunnerbot]

✅ Build karma 34 completed (commit 23033eaa42 by @anthony-redFox)