fix trying to generate RPATH wrappers for Clang

[casparvl]

This loop is supposed to loop over all compilers for which wrappers need to be created. However, this line returns (['clang', 'clang++'],[None, None, None]) for the Clang toolchain. The subsequent call to which happening here then fails.

This PR fixes that by explicitely checking if cmd is None. If so, it proceeds to the next loop iteration and doens't try to create an RPATH wrapper for this 'None' command.

[boegel]

@casparvl It would be nice if we could enhance the test_toolchain_prepare_rpath test to cover this fix.

Without this change, what happens? A hard crash, or RPATH linking just doesn't happen?

[casparvl]

Yes, without this, a prepare_rpath_wrappers() call on a Clang toolchain will fail (some complaint about a Nonetype blabla since it expects a string)

[casparvl]

@boegel is this more or less what you had in mind in terms of test? Can you review this PR, in conjuction with this EasyBlock PR easybuilders/easybuild-easyblocks#2799 ? From my perspective, they are ready for review.

[boegel]

@casparvl That test is perfect: if I run it after temporarily undoing the fix in easybuild/tools/toolchain/toolchain.py, then it reproduces the problem being fixed:

ERROR: test_toolchain_prepare_rpath (test.framework.toolchain.ToolchainTest)
Test toolchain.prepare under --rpath
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Volumes/work/easybuild-framework/test/framework/toolchain.py", line 2627, in test_toolchain_prepare_rpath
    tc_clang.prepare_rpath_wrappers()
  File "/Volumes/work/easybuild-framework/easybuild/tools/toolchain/toolchain.py", line 994, in prepare_rpath_wrappers
    orig_cmd = which(cmd)
  File "/Volumes/work/easybuild-framework/easybuild/tools/filetools.py", line 521, in which
    cmd_path = os.path.join(path, cmd)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/posixpath.py", line 90, in join
    genericpath._check_arg_types('join', a, *p)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/genericpath.py", line 152, in _check_arg_types
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'

[boegel]

lgtm