module: do not attempt to strip type when there's no source

lib/internal/modules/esm/get_format.js

@@ -161,8 +161,12 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
       default: { // The user did not pass `--experimental-default-type`.
         // `source` is undefined when this is called from `defaultResolve`;
         // but this gets called again from `defaultLoad`/`defaultLoadSync`.
-        const { tsParse } = require('internal/modules/helpers');
-        const parsedSource = tsParse(source);
+        let parsedSource;
+        if (source) {
+          // We do the type stripping only if `source` is not falsy.
+          const { tsParse } = require('internal/modules/helpers');
+          parsedSource = tsParse(source);
+        }
         const detectedFormat = detectModuleFormat(parsedSource, url);
         // When source is undefined, default to module-typescript.
         const format = detectedFormat ? `${detectedFormat}-typescript` : 'module-typescript';

lib/internal/modules/helpers.js

@@ -331,8 +331,7 @@ function loadTypeScriptParser(parser) {
  * @returns {string} JavaScript code.
  */
 function tsParse(source) {
-  // TODO(@marco-ippolito) Checking empty string or non string input should be handled in Amaro.
-  if (!source || typeof source !== 'string') { return ''; }
+  assert(typeof source === 'string');
   const parse = loadTypeScriptParser();
   const { code } = parse(source);
   return code;