Remove getJSToNativeConversionTemplate's initialValue return value.

This commit does not change the generated code.
Move the assignment of 'None' in the no-argument-passed case into an else branch.

This is the only case where we assign into an argument local twice, so removing
it will allow us to make that binding immutable.
Move the assignment outside the if when dealing with optional arguments.
Assign into the argument binding directly in instantiateJSToNativeConversionTemplate.