In RichText Editor, you can create line breaks (inside a paragraph) by pressing Shift + Enter.
If you create a table using the rich text field type and type some text in a table cell then add a line break and then add some more text after saving the line break is removed.
If you add a new paragraph in the table cell by pressing Enter then delete the second line with Backspace and then you do the text + line break + text as I've written in the previous line the line break is kept.

It seems like that line breaks are kept only if they are inside a paragraph (p tag) in a table cell.

Steps to reproduce:
1. Create a table in a rich text field.
2. Click in a table cell
3. Type "first line", press Shift+Enter, type "second line"
4. Publish the content
5. View / edit the content, no br tag between "first line" and "second line"

Extra info:
1. The "Steps to reproduce" create a markup like this in the editor (step 1-3):

<td id="yui_3_18_1_1_1520515355833_79125">
first line
<br>
second line
<br>
</td>

2. If you click in an empty cell, press Enter to add new paragraph, then Backspace to delete the second paragraph then you do step 3 it creates a markup in the editor like this:

<td id="yui_3_18_1_1_1520515355833_79227">
<p>
first line
<br>​​​​​​​
second line
<br>
</p>
</td>

This way the line break (br) is kept.

Expected: br should be a valid element inside table cell without being inside paragraph tag.