Fix fold literal encoding with trailing line break

[kekavc24]

This PR:

• Makes _tryYamlEncodeFolded and _tryYamlEncodeLiteral return null for strings with trailing line-breaks
• Add test cases mistakenly removed in Fix line folding #87

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[jonasfj]

This makes things better, no doubt!

Let's land it.

I suppose it's not unthinkable that we can do better than this, but if we want to pursue that we definitely have to consider what the cost will be in terms of complexity vs value.

The fix in this PR is certainly not complex :)