Post category listing table admin: Need to replace blank space with "No description" when description column is blank

[pratikgandhi]

When you see the post category list table from admin panel you will find that the description column shows blank space instead of "text message" which helps to indicate that the description is not available.

I have attached the screenshot for more clarification.

​http://www.awesomescreenshot.com/image/2455447/e13c74d4c1489ff1f6806c4a0b7aac42

So, I have replaced the blank space with the _x('No description');

[pratikgandhi]

Updated description text

[afercia]

See also #40628.

[yahil]

@afercia I have made changes on existing .path to improve coding standard.

Solve phpcs notice:

1. Whitespace found at end of line
2. expected 1 space before/after closing bracket [ _x( 'Unknown' ); ]
3. line indented incorrectly

[SergeyBiryukov]

In 40957:

Administration: In taxonomy list tables, display a dash instead of a blank space and add "No description" as a screen reader text when the term description is blank.

Props pratikgandhi, yahil.
Fixes #40659.

[SergeyBiryukov]

In 40960:

Administration: In WP_List_Table::comments_bubble(), replace the mdash with its HTML entity for consistency with the similar code in other list tables.

Props afercia.
See #40659, #32152.