Generated typescript types not correctly reflecting null option when Node is optional

[ashley-taylor]

Given a gramma that has a parser rule like the following

atom
   : NUMBER
   | VARIABLE

This will generate the following AtomContext

export declare class AtomContext extends ParserRuleContext {
    constructor(parser?: CalculatorSyntaxParser, parent?: ParserRuleContext, invokingState?: number);
    NUMBER(): TerminalNode;
    VARIABLE(): TerminalNode;
    get ruleIndex(): number;
    enterRule(listener: CalculatorSyntaxListener): void;
    exitRule(listener: CalculatorSyntaxListener): void;
}

This does not reflect the optional nature of NUMBER and VARIABLE as those can be null
It should generate a context like the following instead

export declare class AtomContext extends ParserRuleContext {
    constructor(parser?: CalculatorSyntaxParser, parent?: ParserRuleContext, invokingState?: number);
    NUMBER(): TerminalNode | null;
    VARIABLE(): TerminalNode | null;
    get ruleIndex(): number;
    enterRule(listener: CalculatorSyntaxListener): void;
    exitRule(listener: CalculatorSyntaxListener): void;
}