[mlir][py] Fix nanobind uninitialized values (#148944) After #143866, we no longer always write to `value`, causing it to be uninitialized. This can lead to mysterious crashes, e.g. in `python_test.py` / `testCustomAttribute` when we attempt to evaluate `TestAttr(42)`, it does not set `value`, but `mlirAttributeIsNull(value)` happens to return false for garbage memory, and we end up trying to interpret it as a function instead of skipping it. Fix this by only reading `value` if it has been assigned. If it hasn't, `return false` seems the right choice for all these methods, i.e. indicate that `from_python` failed.
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