[mlir][Python] Fix generation of accessors for Optional

Previously, in case there was only one `Optional` operand/result within
the list, we would always return `None` from the accessor, e.g., for a
single optional result we would generate:

```
return self.operation.results[0] if len(self.operation.results) > 1 else None
```

But what we really want is to return `None` only if the length of
`results` is smaller than the total number of element groups (i.e.,
the optional operand/result is in fact missing).

This commit also renames a few local variables in the generator to make
the distinction between `isVariadic()` and `isVariableLength()` a bit
more clear.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D113855