Don't show subprocess output if there isn't any

IPython/testing/iptest.py

@@ -426,12 +426,16 b' class SubprocessStreamCapturePlugin(Plugin):'
     def formatFailure(self, test, err):
         # Show output
         ec, ev, tb = err
+        captured = self.stream_capturer.get_buffer().decode('utf-8', 'replace')
+        if captured.strip():
         ev = safe_str(ev)
         out = [ev, '>> begin captured subprocess output <<',
-                self.stream_capturer.get_buffer().decode('utf-8', 'replace'),
+                    captured,
                 '>> end captured subprocess output <<']
         return ec, '\n'.join(out), tb
     
+        return err
+    
     formatError = formatFailure
     
     def finalize(self, result):