Process check breaks on v2.0.0 of psutil

[conorbranagan]

2.0.0 was just released and has some backward-incompatible changes. Traceback of one of the issues:

- instance #2 [ERROR]: TypeError('',)
        Traceback (most recent call last):
          File "/usr/share/datadog/agent/checks/__init__.py", line 461, in run
            self.check(instance)
          File "/usr/share/datadog/agent/checks.d/process.py", line 177, in check
            pids = self.find_pids(search_string, psutil, exact_match=exact_match)
          File "/usr/share/datadog/agent/checks.d/process.py", line 53, in find_pids
            if string in ' '.join(proc.cmdline):
        TypeError