Fix method handler control flow in Windows and Linux text input plugins

[stuartmorgan-g]

The existing logic incorrectly factored out a check that there were arguments too early, applying it to any message not already handled (including unhandled methods, such as methods added after the initial implementation) and thus failing if any unhandled message had no arguments.

Fixes flutter/flutter#55653

[stuartmorgan-g]

The diff is almost entirely whitespace, so it's not nearly as much change as it looks like; the substantive change is flattening the nested if/else if chain, and duplicating the argument extraction into the two method handlers that need it.

[stuartmorgan-g]

@robert-ancell FYI in case you were using this code as a model for the GTK code.

[stuartmorgan-g]

@chinmaygarde Ping on this review.