refactor(compiler-cli): function declarations must have names (angular#38866)

The `AstFactory.createFunctionDeclaration()` was allowing `null` to be
passed as the function `name` value. This is not actually possible, since
function declarations must always have a name.

PR Close angular#38866

Author: petebacondarwin

packages/compiler-cli/src/ngtsc/translator/src/api/ast_factory.ts

@@ -97,7 +97,7 @@ export interface AstFactory<TStatement, TExpression> {
    * @param parameters the names of the function's parameters.
    * @param body a statement (or a block of statements) that are the body of the function.
    */
-  createFunctionDeclaration(functionName: string|null, parameters: string[], body: TStatement):
+  createFunctionDeclaration(functionName: string, parameters: string[], body: TStatement):
       TStatement;
 
   /**

packages/compiler-cli/src/ngtsc/translator/src/typescript_ast_factory.ts

@@ -79,13 +79,13 @@ export class TypeScriptAstFactory implements AstFactory<ts.Statement, ts.Express
 
   createExpressionStatement = ts.createExpressionStatement;
 
-  createFunctionDeclaration(functionName: string|null, parameters: string[], body: ts.Statement):
+  createFunctionDeclaration(functionName: string, parameters: string[], body: ts.Statement):
       ts.Statement {
     if (!ts.isBlock(body)) {
       throw new Error(`Invalid syntax, expected a block, but got ${ts.SyntaxKind[body.kind]}.`);
     }
     return ts.createFunctionDeclaration(
-        undefined, undefined, undefined, functionName ?? undefined, undefined,
+        undefined, undefined, undefined, functionName, undefined,
         parameters.map(param => ts.createParameter(undefined, undefined, undefined, param)),
         undefined, body);
   }