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/language-service/src/completions.ts

@@ -65,7 +65,7 @@ function getBoundedWordSpan(
     // The HTML tag may include `-` (e.g. `app-root`),
     // so use the HtmlAst to get the span before ayazhafiz refactor the code.
     return {
-      start: templateInfo.template.span.start + ast.startSourceSpan!.start.offset + 1,
+      start: templateInfo.template.span.start + ast.startSourceSpan.start.offset + 1,
       length: ast.name.length
     };
   }