Fix clang test failures from Any.cast

[dsobek]

Fixes #939
Only results in one less detailed error message (but otherwise the same failure conditions are still covered) and fixes the clang build.

To test build in clang and run unit tests. They should now pass.

CC=clang CXX=clang++ colcon build
./build/behaviortree_cpp/tests/behaviortree_cpp_test

[facontidavide]

thanks, but I think that I have found a better solution. See #941