The IDL generator was passing 'false' to generate_to_cpp when converting
a union containing a string type.
This broke WebIDL's LegacyNullToEmptyString behavior.
Pass the actual legacy_null_to_empty_string flag instead.
Fixes some WPTs:
domparsing/outerhtml-02
trusted-types/block-string-assignment-to-Element-outerHTML
trusted-types/block-string-assignment-to-HTMLElement-generic
trusted-types/block-string-assignment-to-ShadowRoot-innerHTML
