Fixes to NPE errors when searching directories recursively

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

@@ -16,10 +16,12 @@ abstract class Engine extends Logging{
 
   def getAllImageFiles(directoryPath:String, recursive:Boolean=false, recursiveDepth:Int=500):List[File] = {
     val fileList = new mutable.MutableList[File]()
+    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) {
@@ -30,6 +32,8 @@ abstract class Engine extends Logging{
             }
           }
         }
+      }
+    }
     fileList.toList
   }