Refine handling of null sources in various places

When a source is null, it won't be included in the file map and checks that use
a file won't be run. In the case that a sourcesContent is provided, it would
potentially be better to treat that as an anonymous file but we currently don't.

Also check for the string "null" in some cases, as the underlying source maps
library can return this instead of a null.

src/util/collectSourceFiles.ts

@@ -5,8 +5,11 @@ import type { SourceMap } from "./sourceMap.js";
 export function collectSourceFiles(sourceMap: SourceMap, originalFolderPath: string) {
   const filesMap = new Map<string, TestingFile>();
 
-  sourceMap.sources.forEach((file: string, index: number) => {
-    filesMap.set(file, TestingFile.forTextFile(path.join(originalFolderPath, file), sourceMap.sourcesContent ? sourceMap.sourcesContent[index] : null))
+  sourceMap.sources.forEach((file: string | null, index: number) => {
+    // When the source is null, it might make sense to map to an anonymous source
+    // if a sourceContent is present, but for now just don't add it to the set.
+    if (file !== null)
+      filesMap.set(file, TestingFile.forTextFile(path.join(originalFolderPath, file), sourceMap.sourcesContent ? sourceMap.sourcesContent[index] : null))
   });
 
   return filesMap;

src/validators/SourceFilesValidator.ts

@@ -12,8 +12,12 @@ export class SourceFilesValidator extends Validator {
     function check(sources : any) {
       sources.forEach((sourceFileName: string | null, index: number) => {
         const fullPath = sourceFileName === null ? null : path.join(context.originalFolderPath, sourceFileName);
-        if ((!fullPath || !fs.existsSync(fullPath)) && sourcesContent[index] == undefined) {
-          errors.push(new Error(`Source file not found: ${sourceFileName} ${sourcesContent[index]}`));
+        // If the path is null, we won't use the source in subsequent passes so
+        // it can be ignored. Otherwise ensure the source makes sense.
+        if (fullPath !== null) {
+          if ((!fullPath || !fs.existsSync(fullPath)) && sourcesContent[index] == undefined) {
+            errors.push(new Error(`Source file not found: ${sourceFileName} ${sourcesContent[index]}`));
+          }
         }
       });
     };

src/validators/SourceMapMappingsValidator.ts

@@ -23,7 +23,7 @@ export class SourceMapMappingsValidator extends Validator {
       await SourceMapConsumer.with(sourceMap, null, (consumer) => {
         consumer.eachMapping((mapping) => {
           // Is this a valid situation following the spec?
-          if (mapping.source === null) return
+          if (mapping.source === null || mapping.source === "null") return
           const originalFile = originalFiles.get(mapping.source);
 
           if (originalFile === undefined) {