Fix single entry VPSet template bug

If a VPSet had a single entry and defined a `template` then a `dumpPython` would not create a syntactically correct output.

Author: Dr15Jones

FWCore/ParameterSet/python/Mixins.py

@@ -697,7 +697,9 @@ def dumpPython(self, options:PrintOptions=PrintOptions()) -> str:
         if n>=256:
             #wrap in a tuple since they don't have a size constraint
             result+=" ("
+        wroteAtLeastOne = False
         for i, v in enumerate(self):
+            wroteAtLeastOne = True
             if i == 0:
                 if n>nPerLine: result += '\n'+options.indentation()
             else:
@@ -710,7 +712,7 @@ def dumpPython(self, options:PrintOptions=PrintOptions()) -> str:
             result +=' ) '
         moreArgs = self._additionalInitArguments(options)
         if moreArgs:
-            if i > 0:
+            if wroteAtLeastOne:
                 result += ', \n' + options.indentation()
             result += moreArgs
         if n>nPerLine: