Merge pull request #11634 from vpisarev:empty_mat_with_types_2

fixes handling of empty matrices in some functions (#11634)

* a part of PR #11416 by Yuki Takehara

* moved the empty mat check in Mat::copyTo()

* fixed some test failures

modules/features2d/src/agast.cpp

@@ -7952,6 +7952,12 @@ public:
     {
         CV_INSTRUMENT_REGION()
 
+        if(_image.empty())
+        {
+            keypoints.clear();
+            return;
+        }
+
         Mat mask = _mask.getMat(), grayImage;
         UMat ugrayImage;
         _InputArray gray = _image;

modules/features2d/src/fast.cpp

@@ -526,6 +526,12 @@ public:
     {
         CV_INSTRUMENT_REGION()
 
+        if(_image.empty())
+        {
+            keypoints.clear();
+            return;
+        }
+
         Mat mask = _mask.getMat(), grayImage;
         UMat ugrayImage;
         _InputArray gray = _image;

modules/features2d/src/gftt.cpp

@@ -80,6 +80,12 @@ public:
     {
         CV_INSTRUMENT_REGION()
 
+        if(_image.empty())
+        {
+            keypoints.clear();
+            return;
+        }
+
         std::vector<Point2f> corners;
 
         if (_image.isUMat())