Merge remote-tracking branch 'upstream/3.4' into merge-3.4

Revert "documentation: avoid links to 'master' branch from 3.4 maintenance branch"
This reverts commit 9ba9358ecb.

Revert "documentation: avoid links to 'master' branch from 3.4 maintenance branch (2)"
This reverts commit f185802489.

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

@@ -522,6 +522,12 @@ 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())