Fixes to NPE errors when searching directories recursively

src/main/scala/com/sothr/imagetools/Engine.scala

@@ -16,17 +16,21 @@ abstract class Engine extends Logging{
 
   def getAllImageFiles(directoryPath:String, recursive:Boolean=false, recursiveDepth:Int=500):List[File] = {
     val fileList = new mutable.MutableList[File]()
-    val directory:File = new File(directoryPath)
-    val imageFilter = new ImageFilter
-    if (directory.isDirectory) {
-      val files = directory.listFiles(imageFilter)
-      fileList ++= files
-      info(s"Found ${files.length} files that are images in directory: $directoryPath")
-      if (recursive) {
-        val directoryFilter = new DirectoryFilter
-        val directories = directory.listFiles(directoryFilter)
-        for (directory <- directories) {
-            fileList ++= getAllImageFiles(directory.getAbsolutePath, recursive, recursiveDepth-1)
+    if (directoryPath != null && directoryPath != "") {
+      val directory:File = new File(directoryPath)
+      val imageFilter = new ImageFilter
+      if (directory.isDirectory) {
+        val files = directory.listFiles(imageFilter)
+        if (files != null) {
+          fileList ++= files
+          info(s"Found ${files.length} files that are images in directory: $directoryPath")
+          if (recursive) {
+            val directoryFilter = new DirectoryFilter
+            val directories = directory.listFiles(directoryFilter)
+            for (directory <- directories) {
+                fileList ++= getAllImageFiles(directory.getAbsolutePath, recursive, recursiveDepth-1)
+            }
+          }
         }
       }
     }