Ability to continue to Shift after a Transpose element.

Interesting and powerful feature; for an example see : https://raw.githubusercontent.com/bazaarvoice/jolt/f8f18e8544f7f3ffef8c5a8f9e059a844857387f/jolt-core/src/test/resources/json/shiftr/transposeNestedLookup.json

Additionally, "fixed" an issue where Shiftr could not differentiate between something not existing or a legitimate null in the input data.s

However this necessitated two small backward incompatibilities

1. Low level Java interfaces used by Shiftr to output data, generally got updated to have an Optional return type.
2. If you had specs that relied upon the behavior of not passing legitimate null, then you will need to do some work.