USER in prompts default to '' instead of None

IPython/core/prompts.py

@@ -140,7 +140,7 b' HOME = py3compat.str_to_unicode(os.environ.get("HOME","//////:::::ZZZZZ,,,~~~"))'
 # We precompute a few more strings here for the prompt_specials, which are
 # fixed once ipython starts.  This reduces the runtime overhead of computing
 # prompt strings.
-USER           = py3compat.str_to_unicode(os.environ.get("USER"))
+USER           = py3compat.str_to_unicode(os.environ.get("USER",''))
 HOSTNAME       = py3compat.str_to_unicode(socket.gethostname())
 HOSTNAME_SHORT = HOSTNAME.split(".")[0]
 ROOT_SYMBOL    = "#" if (os.name=='nt' or os.getuid()==0) else "$"