Use default value when fetching prefix+ '_URL'

When a driver does not register an environment variable sufixed
with _URL, pifpaf will break having None when it expets a string.

Signed-off-by: Moisés Guimarães de Medeiros <guimaraes@pm.me>

pifpaf/__main__.py

@@ -185,7 +185,7 @@ def expand_urls_var(url):
 
             putenv("PID", str(os.getpid()))
             putenv("DAEMON", daemon)
-            url = os.getenv(driver.env_prefix + "_URL")
+            url = os.getenv(driver.env_prefix + "_URL", "")
             putenv("%s_URL" % daemon.upper(), url)
             os.putenv(global_urls_variable,
                       expand_urls_var(url))
@@ -251,7 +251,7 @@ def _cleanup(signum, frame):
                 signal.signal(signal.SIGPIPE, signal.SIG_IGN)
                 signal.pause()
             else:
-                url = driver.env['%s_URL' % driver.env_prefix]
+                url = driver.env.get('%s_URL' % driver.env_prefix, "")
                 driver.env.update({
                     "PIFPAF_PID": pid,
                     env_prefix + "_PID": pid,