add more test cases (#8072)

Author: sissbruecker

vaadin-upload-flow-parent/vaadin-upload-flow/src/test/java/com/vaadin/flow/component/upload/tests/UploadHandlerTest.java

@@ -113,6 +113,23 @@ public void setUploadHandler_withCustomTarget_generatedUrlEndsWithCustomName() {
                 targetUploadUrl.endsWith(targetName));
     }
 
+    @Test
+    public void setUploadHandler_withNullTarget_throws() {
+        Assert.assertThrows(NullPointerException.class,
+                () -> upload.setUploadHandler(event -> {
+                }, null));
+    }
+
+    @Test
+    public void setUploadHandler_withBlankTarget_throws() {
+        Assert.assertThrows(IllegalArgumentException.class,
+                () -> upload.setUploadHandler(event -> {
+                }, ""));
+        Assert.assertThrows(IllegalArgumentException.class,
+                () -> upload.setUploadHandler(event -> {
+                }, "   "));
+    }
+
     @Test
     public void uploadWithStandardHandler_activeUploadsIsChanged_from0to1_from1to0()
             throws IOException, URISyntaxException {