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/localize/src/tools/src/translate/translation_files/translation_parsers/translation_utils.ts

@@ -48,7 +48,7 @@ export function parseInnerRange(element: Element): ParseTreeResult {
  * @param element The element whose inner range we want to compute.
  */
 function getInnerRange(element: Element): LexerRange {
-  const start = element.startSourceSpan!.end;
+  const start = element.startSourceSpan.end;
   const end = element.endSourceSpan!.start;
   return {
     startPos: start.offset,