fix error in ANIm coverage dataframe update

When subject cover was zero, the dataframe containing coverage
values was not being updated because of a test for `if scover`
instead of `if scover is not None`.

pyani/anim.py

@@ -221,8 +221,10 @@ def process_deltadir(delta_dir, org_lengths, logger=None):
                 logger.warning(
                     "Total alignment length reported in " + "%s is zero!" % deltafile
                 )
+        print(f"{tot_length=} {org_lengths[qname]=} {org_lengths[sname]=}")
         query_cover = float(tot_length) / org_lengths[qname]
         sbjct_cover = float(tot_length) / org_lengths[sname]
+        print(f"{query_cover=} {sbjct_cover=}")
 
         # Calculate percentage ID of aligned length. This may fail if
         # total length is zero.
@@ -240,5 +242,8 @@ def process_deltadir(delta_dir, org_lengths, logger=None):
         results.add_tot_length(qname, sname, tot_length)
         results.add_sim_errors(qname, sname, tot_sim_error)
         results.add_pid(qname, sname, perc_id)
+
+        print(f"before adding coverage {results.alignment_coverage=}")
         results.add_coverage(qname, sname, query_cover, sbjct_cover)
+        print(f"after adding coverage {results.alignment_coverage=}")
     return results

pyani/pyani_tools.py

@@ -83,7 +83,7 @@ def add_pid(self, qname, sname, value, sym=True):
     def add_coverage(self, qname, sname, qcover, scover=None):
         """Add percentage coverage values to self.alignment_coverage."""
         self.alignment_coverage.loc[qname, sname] = qcover
-        if scover:
+        if scover is not None:  # need to account for coverage being zero
             self.alignment_coverage.loc[sname, qname] = scover
 
     @property