When coping data from DataGridView, there should no longer apply blank unselected lines onto the clipboard

* .NET Core Version: .NET SDK 5.0.100-preview.3.20180.10

* Have you experienced this same bug with .NET Framework?: Yes

**Problem description:**
When coping data from DataGridView, blank unselected lines were applied onto the clipboard.

**Actual behavior:**
Unselected cells in these rows or columns are copied as blank placeholders to the Clipboard.

**Expected behavior:**
There should no longer applies blank unselected lines onto the clipboard.

**Minimal repro:**
1. Create a .NET Core winforms application.
2. Add a DataGridView with some columns to the form designer and run the app.
3. Add some data to the rows of DataGridView.
4. Select and copy the first and third rows, then paste them to a notepad.

**App:** [SampleApp.zip](https://github.com/dotnet/winforms/files/4406524/SampleApp.zip)