Allow no input tensor

As strange as it may be, there are a lot of cases where we will generate everything inside our block. Stop enforcing that a tensor be injected, and allow nothingness in this place.

Start by locating the validation that requires an input tensor and inspect how blocks are constructed when inputs are supplied. Check nearby callers or examples to understand the expected no-input case. Done means a block can generate its contents without an injected tensor while existing input-driven behavior still works.

Written by the indexing model from the issue text.