[Flow][Stream] Fix assertions in build function (#23058)

Relaxes assertions in the `build` functions for `flow.func` and
`stream.async.func`.

The `FunctionOpTrait` only requires:
- If argument attributes are present, their number must match the number
of arguments.
- If result attributes are present, their number must match the number
of results.

Due to the structure of the code, the `assert`s so far required the
number of argument attributes to match the number of arguments when
result attributes are present, even if no argument attributes were
specified (and vice versa for results).

This leads to assertion errors for legal functions which specify a full
set of argument attributes but no result attributes at all, or vice
versa.

This is based on bugs reported from fuzzing, it fixes
https://github.com/iree-org/iree/issues/22962 and fixes
https://github.com/iree-org/iree/issues/22961.

Signed-off-by: Lukas Sommer <lukas.sommer@amd.com>