Avoid a potential http request if there are no services

The vc_export_env call was introduced in commit
37ca553 ("Export vc env vars when
running a source service") and broke the testsuite. The potential
http request is useless if there are no services. Hence, avoid it.

Thanks to Marco Strigl for catching the testsuite error!

osc/core.py

@@ -406,6 +406,12 @@ def execute(self, dir, callmode = None, singleservice = None, verbose = None):
             data = { 'name' : singleservice, 'command' : [ singleservice ], 'mode' : '' }
             allservices = [data]
 
+        if not allservices:
+            # short-circuit to avoid a potential http request in vc_export_env
+            # (if there are no services to execute this http request is
+            # useless)
+            return 0
+
         # services can detect that they run via osc this way
         os.putenv("OSC_VERSION", get_osc_version())