Improve readability of manufacturer_info_field()

Fixes issue #129, but originally suggested in review of #121.

src/wireviz/wv_helper.py

@@ -123,7 +123,7 @@ def open_file_append(filename):
     return open(filename, 'a', encoding='UTF-8')
 
 def manufacturer_info_field(manufacturer, mpn):
-    if manufacturer or mpn:
-        return f'{manufacturer if manufacturer else "MPN"}{": " + str(mpn) if mpn else ""}'
+    if manufacturer:
+        return f'{manufacturer}: {mpn}' if mpn else manufacturer
     else:
-        return None
+        return f'MPN: {mpn}' if mpn else None