(fix #5290) Support empty input in TrasnformOverride.ofSource

[clairemcginty]

unfortunately this requires changing the signature to require a Coder.

[clairemcginty]

The rest of this diff is just rearranging the tests to be grouped by API:

[info] JobTest
...
[info] transformOverride.ofSource
[info] - should support non-empty input (69 milliseconds)
[info] - should support empty input (78 milliseconds)
[info] TransformOverride.ofIter
[info] - should support 1-to-n inputs (147 milliseconds)
[info] - should support 1-to-n function inputs (56 milliseconds)
[info] TransformOverride.of
[info] - should support non-empty input (66 milliseconds)
[info] - should support function input (56 milliseconds)
[info] - should fail with an incorrect override input type (27 milliseconds)
[info] - should fail with an incorrect function override input type (40 milliseconds)
[info] - should fail differently with an incorrect override generic input type (69 milliseconds)
[info] - should fail with an incorrect override output type (30 milliseconds)
[info] TransformOverride.ofKV
[info] - should support non-empty input (56 milliseconds)
[info] - should suppport a KV function override (50 milliseconds)
[info] TransformOverride.ofIterKV
[info] - should support non-empty input (60 milliseconds)
[info] - should support a 1-to-n KV function input (57 milliseconds)
[info] TransformOverride.ofAsyncLookup
[info] - should support non-empty input (55 milliseconds)
[info] - should pass with an AsyncLookup function override (55 milliseconds)
[info] TransformOverride.ofIterAsyncLookup
[info] - should support a 1-to-n AsyncLookup function override (53 milliseconds)
[info] - should support a 1-to-n AsyncLookup override (55 milliseconds)

[RustedBones]

I think this is still worth adding this breaking change in the next 0.14 patch. Let's add a filtering to the mima