bug fix for signing a doclet with source = [ undefined ]

[LeeJenkins]

This is a fix for a bug I discovered.

Background
I have been running classy for several weeks. It worked great. Loved the page formatting.

Then one day I noticed that it was no longer regenerating the documentation files.

I traced the problem to class DocletPage in static sign(doclet) at this line of code:

        const source = doclet.yields || doclet.returns || [];

One of my files somehow causes source to be assigned to [ undefined ]. I did not backtrace how that happens, but it does.

Symptom
If the source array contains an undefined element, then the process terminates without printing any error.

Root Cause
I tracked the problem into this call to PublishUtils.typeStrings(), which itself calls helper functions with undefined.

            const returns = source.map(s => s.type?.names).flat()
                .map(s => PublishUtils.typeStrings({type: {names: [s]}})).join("|");

Fix
I changed the source array pre-processing to filter undefined elements using .filter(s => s):

            const returns = source.filter(s => s).map(s => s.type?.names).flat()
                .map(s => PublishUtils.typeStrings({type: {names: [s]}})).join("|");

With this change in place, jsdoc/classy properly generate the documentation files.