fix(schema): avoid returning string 'nested' as schematype

[vkarpov15]

Fix #14435
Fix #14443

@FaizBShah

Summary

A more "root cause" fix for #14435 than #14443. I think it is incorrect for _getPath() to ever return a string: we only want SchemaType instance or nullish. The issue comes from us setting nested paths to 'nested' for nested paths underneath single nested subdocs - if we don't have that, then the test for #9459 starts failing.

Examples

[FaizBShah]

@vkarpov15 This looks good to me. Just one query, currently according to the docs, function like schema.pathType() returns 'nested' for nested paths. So, will this change affect that or some other functions?

[hasezoey]

unless i missed something, arent the 2 added tests basically the same?

[vkarpov15]

Not quite, 2nd test is slightly different. But as far as I can tell, the 2nd test is also unnecessary, it passes both with and without the code changes in this PR.

[FaizBShah]

unless i missed something, arent the 2 added tests basically the same?

@hasezoey No, in the first test the extras field was an array, while in the 2nd test, the extras field was a Schema field. The reason I wrote the 2 tests was because I saw that the issue was working when it was just a schema field, but failing when it was an array. So I wrote the tests to ensure both passes. And I didn't wrote .create() because the original issuer who gave the repro script didn't have it, but I think the changes done in this PR makes better sense