[fix] node.clean - ignore npm dependencies when npm is not installed

error pattern::

    $ make clean
    CLEAN     pyenv
    PYENV     [virtualenv] drop local/py3
    CLEAN     docs -- build/docs dist/docs
    CLEAN     locally installed npm dependencies
    ./manage: line 318: npm: command not found
    ERROR: node.clean exit with error (127)
    make: *** [Makefile:90: node.clean] Error 127

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>