Fix SWHIndexingLister.db_partition_indices(); ensure partition size is not zero

when this lister is called for the first time, db_num_entries() may return
a null value, so the min() will also be zero, making the range() call crash.

swh/lister/core/indexing_lister.py

@@ -103,7 +103,7 @@ class SWHIndexingLister(SWHListerBase):
                 declare approximately equal-sized ranges of existing
                 repos
         """
-        n = self.db_num_entries()
+        n = max(self.db_num_entries(), 10)
 
         partitions = []
         partition_size = min(partition_size, n)