Handle findSourceMap returning null since Node.js 18.8.0

Co-authored-by: Mark Wubben <[email protected]>

Author: Brooooooklyn

lib/snapshot-manager.js

@@ -391,7 +391,9 @@ class Manager {
 
 const resolveSourceFile = mem(file => {
 	const sourceMap = findSourceMap(file);
-	if (sourceMap === undefined) {
+	// Prior to Node.js 18.8.0, the value when a source map could not be found was `undefined`.
+	// This changed to `null` in <https://github.com/nodejs/node/pull/43875>. Check both.
+	if (sourceMap === undefined || sourceMap === null) {
 		return file;
 	}