Fix bug when writing target stats without predict activity model

When running in complete-targets mode without an activity model set
(e.g., for the minimize-guides objective), target_search.py would try to
compute expected activities for each guide. Doing so would raise an
error. This commit fixes that by using nan for the expected activity of
each guide in this case, as is the case for other activity prediction
stats.

adapt/target_search.py

@@ -545,10 +545,18 @@ def find_and_write_targets(self, out_fn, best_n=10):
                 # Find expected activity for each guide
                 window_start = p1.start + p1.primer_length
                 window_end = p2.start
-                expected_activities_per_guide = \
-                        [self.gs.guide_activities_expected_value(
-                            window_start, window_end, gd_seq)
-                            for gd_seq in guides_seqs_sorted]
+                if self.gs.predictor is not None:
+                    expected_activities_per_guide = \
+                            [self.gs.guide_activities_expected_value(
+                                window_start, window_end, gd_seq)
+                                for gd_seq in guides_seqs_sorted]
+                else:
+                    # There is no predictor to predict activities
+                    # This should only be the case if self.obj_type is 'min',
+                    # and may not necessarily be the case if self.obj_type is
+                    # 'min'
+                    expected_activities_per_guide = \
+                            [math.nan for gd_seq in guides_seqs_sorted]
                 expected_activities_per_guide_str = ' '.join(
                         str(a) for a in expected_activities_per_guide)