lib/oe/distro_check: don't set empty proxy keys

lib/oe/distro_check: don't set empty proxy keys

If the proxies dictionary has a proxy set to None urllib will throw an exception
instead of not using a proxy (abridged stack):

File: '/home/ross/Yocto/poky/meta/lib/oe/distro_check.py', lineno: 43, function: get_links_from_url
 *** 0043:    with create_socket(url,d) as sock:
     0044:        webpage = sock.read()
File: '/home/ross/Yocto/poky/meta/lib/oe/distro_check.py', lineno: 5, function: create_socket
     0003:def create_socket(url, d):
     0004:    import urllib
 *** 0005:    socket = urllib.urlopen(url, proxies=get_proxies(d))
File: '/usr/lib/python2.7/urllib.py', lineno: 87, function: urlopen
     0086:    if data is None:
 *** 0087:        return opener.open(url)
File: '/usr/lib/python2.7/urllib.py', lineno: 203, function: open
     0201:        else:
     0202:            proxy = None
 *** 0203:        name = 'open_' + urltype
     0204:        self.type = urltype
     0205:        name = name.replace('-', '_')
Exception: TypeError: cannot concatenate 'str' and 'NoneType' objects

Filter out unset values so that the dictionary only has valid assignments in.

Signed-off-by: Ross Burton <ross.burton@intel.com>