Fix test for volume source spaces with restricted orientations

mne/minimum_norm/inverse.py

@@ -1407,7 +1407,7 @@ def _prepare_forward(forward, info, noise_cov, fixed, loose, rank, pca,
             raise ValueError('Computing inverse solutions for volume source '
                              'spaces with fixed orientations is not '
                              'supported.')
-    if 'volume' in loose and any(v < 1 for v in loose.values()):
+    if loose.get('volume', 1) < 1:
         raise ValueError('Computing inverse solutions with restricted '
                          'orientations (loose < 1) is not supported for '
                          'volume source spaces.')