[mlir][linalg][bufferize][NFC] Simplify bufferization of CallOps

There is no need to inspect the ReturnOp of the called function.

This change also refactors the bufferization of CallOps in such a way that `lookupBuffer` is called only a single time. This is important for a later change that fixes CallOp bufferization. (There is currently a TODO among the test cases.)

Note: This change modifies a test case but is marked as NFC. There is no change of functionality, but FuncOps with empty bodies are now reported with a different error message.

Differential Revision: https://reviews.llvm.org/D116446