Better handle empty insertion codes

wrappers/python/simtk/openmm/app/pdbfile.py

@@ -372,7 +372,7 @@ class PDBFile(object):
                     else:
                         atomName = atom.name
                     coords = positions[posIndex]
-                    line = "%s%5d %-4s %3s %s%4s%s   %s%s%s  1.00  0.00          %2s  " % (
+                    line = "%s%5d %-4s %3s %s%4s%1s   %s%s%s  1.00  0.00          %2s  " % (
                         recordName, atomIndex%100000, atomName, resName, chainName, resId, resIC, _format_83(coords[0]),
                         _format_83(coords[1]), _format_83(coords[2]), symbol)
                     assert len(line) == 80, 'Fixed width overflow detected'

wrappers/python/simtk/openmm/app/pdbxfile.py

@@ -385,7 +385,7 @@ class PDBxFile(object):
             for (resIndex, res) in enumerate(residues):
                 if keepIds:
                     resId = res.id
-                    resIC = res.insertionCode
+                    resIC = (res.insertionCode if len(res.insertionCode) > 0 else '.')
                 else:
                     resId = resIndex + 1
                     resIC = '.'