added an ability to make push_back to the empty matrix

modules/core/include/opencv2/core/mat.hpp

@@ -675,6 +675,11 @@ template<typename _Tp, int m, int n> inline Mat::operator Matx<_Tp, m, n>() cons
 
 template<typename _Tp> inline void Mat::push_back(const _Tp& elem)
 {
+    if( !data )
+	{
+		*this = Mat(1, 1, DataType<_Tp>::type, (void*)&elem).clone();
+		return;
+	}
 	CV_Assert(DataType<_Tp>::type == type() && cols == 1
               /* && dims == 2 (cols == 1 implies dims == 2) */);
     uchar* tmp = dataend + step[0];

modules/core/src/matrix.cpp

@@ -585,6 +585,11 @@ void Mat::push_back(const Mat& elems)
         push_back(tmp);
         return;
     }
+	if( !data )
+	{
+		*this = elems.clone();
+		return;
+	}
 
     size.p[0] = elems.size.p[0];
     bool eq = size == elems.size;