[INTERNAL] transformApiJson: Fix regression in UI5 Tooling scenario

(cherry picked from UI5/openui5@d3bcca9)

Author: matz3

lib/processors/jsdoc/lib/transformApiJson.cjs

@@ -989,11 +989,12 @@ function transformer(sInputFile, sOutputFile, sLibraryFile, vDependencyAPIFiles,
 			const {sources} = options;
 			let {librarySrcDir, libraryTestDir} = options;
 
-			// Using path.join to normalize POSIX paths to Windows paths on Windows
-			librarySrcDir = path.join(librarySrcDir);
-			libraryTestDir = path.join(libraryTestDir);
-
 			if (Array.isArray(sources) && typeof librarySrcDir === "string" && typeof libraryTestDir === "string") {
+
+				// Using path.join to normalize POSIX paths to Windows paths on Windows
+				librarySrcDir = path.join(librarySrcDir);
+				libraryTestDir = path.join(libraryTestDir);
+
 				/**
 				 * Calculate prefix to check
 				 *