refactor(compiler): element.startSourceSpan is required (#38581)

Previously, the `startSourceSpan` property could be null
but in reality it is always well defined - except for a legacy
case in the old i18n extraction/merging code, where the
typings for source-spans are already being undermined.

Making this property non-null, simplifies code elsewhere
in the project.

PR Close #38581

packages/compiler/test/template_parser/template_parser_spec.ts

@@ -140,7 +140,7 @@ class TemplateHumanizer implements TemplateAstVisitor {
 
   private _appendSourceSpan(ast: TemplateAst, input: any[]): any[] {
     if (!this.includeSourceSpan) return input;
-    input.push(ast.sourceSpan!.toString());
+    input.push(ast.sourceSpan.toString());
     return input;
   }
 }