fix: SCHWED-5540 Remove null character from insertion code

A null character is added by OST in case of empty insertion code. This is not part of the actual insertion code, and could make it harder for third party software to handle our output.

actions/ost-compare-ligand-structures

@@ -311,7 +311,12 @@ def _Validate(structure, ligands, legend, fault_tolerant=False):
 def _QualifiedResidueNotation(r):
     """Return a parsable string of the residue in the format:
     ChainName.ResidueNumber.InsertionCode."""
-    return f"{r.chain.name}.{r.number.num}.{r.number.ins_code}"
+    resnum = r.number
+    return "{cname}.{rnum}.{ins_code}".format(
+        cname=r.chain.name,
+        rnum=resnum.num,
+        ins_code=resnum.ins_code.strip("\u0000"),
+    )
 
 
 def _Process(model, model_ligands, reference, reference_ligands, args):