Merge pull request #27972 from asmorkalov:as/dst_mat_type

Force empty output type where it's defined in API #27972

The PR replaces:
- https://github.com/opencv/opencv/pull/27936
- https://github.com/opencv/opencv/pull/21059

Empty matrix has undefined type, so user code should not relay on the output type, if it's empty. The PR introduces some exceptions:
- copyTo documentation defines, that the method re-create output buffer and set it's type.
- convertTo has output type as parameter and output type is defined and expected.

modules/core/test/test_umat.cpp

@@ -1199,6 +1199,14 @@ TEST(UMat, async_cleanup_without_call_chain_warning)
     }
 }
 
+TEST(UMat, copyToConvertTo_Empty)
+{
+    cv::UMat A(0, 0, CV_16SC2), B, C;
+    A.copyTo(B);
+    ASSERT_EQ(A.type(), B.type());
+    A.convertTo(C, CV_32SC2);
+    ASSERT_EQ(C.type(), CV_32SC2);
+}
 
 ///////////// oclCleanupCallback threadsafe check (#5062) /////////////////////