Return empty string instead to prevent exception downstream particularly in writeReferencePath

Author: Kanchalai Tanglertsampan

src/compiler/utilities.ts

@@ -2653,7 +2653,7 @@ namespace ts {
             if (sourceFiles.length) {
                 const jsFilePath = options.outFile || options.out;
                 const sourceMapFilePath = getSourceMapFilePath(jsFilePath, options);
-                const declarationFilePath = options.declaration ? removeFileExtension(jsFilePath) + ".d.ts" : undefined;
+                const declarationFilePath = options.declaration ? removeFileExtension(jsFilePath) + ".d.ts" : "";
                 action({ jsFilePath, sourceMapFilePath, declarationFilePath }, createBundle(sourceFiles), emitOnlyDtsFiles);
             }
         }