[executorch] Always generate CustomOpsNativeFunctions.h if custom_ops.yaml is present (#95084)
To match the build system logic, enforce CustomOpsNativeFunctions.h to be generated if we have custom_ops.yaml, even if we don't select any custom ops.
Added unit test.
Differential Revision: [D43402718](https://our.internmc.facebook.com/intern/diff/D43402718)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95084
Approved by: https://github.com/iseeyuan
