EZP-31933: Fixed custom tag link edit

[SerheyDolgushev]

Question	Answer
JIRA issue	EZP-31933
Bug/Improvement	yes
New feature	no
Target version	2.1
BC breaks	no
Tests pass	yes
Doc needed	no

Steps to reproduce

1. Open any RichText field in Online Editor
2. Remove all the field content
3. Insert any custom tag
4. Make sure there are no empty paragraphs before and after the inserted custom tag
5. Edit the custom tag
6. Insert a "test" text
7. Make the inserted text a link
8. Chose any location for the link
9. Click the "Save" link button

Expected result
The link within the custom tag is updated.

Actual result
The link is updated, but Online Editor breaks because of the following JavaScript error:

react-dom.production.min.js:61 Uncaught TypeError: Cannot read property 'lastElement' of null

TODO:

• Implement feature / fix a bug.
• Implement tests.
• Fix new code according to Coding Standards ($ composer fix-cs).
• Ask for Code Review.

[tomaszszopinski]

QA approved on eZPlatform-ee v3.1.2 with diff.

[lserwatka]

@dew326 could you merge it up?

[dew326]

done