Posting accepts blank content (space)

## Bug Report

**Current Behavior**
Typing nothing but spaces in the discussion composer is accepted as proper content by Flarum.

**Steps to Reproduce**
1. Click on the start discussion button
2. Name it whatever
3. Type nothing but a space in post content
4. Submit
5. See error

**Expected Behavior**
I expected the composer to throw an error letting me know that such content is not accepted.

**Screenshots**
No screenshots, but you can see it here: https://discuss.flarum.org/d/19485-blank-content

**Environment**
- Flarum discuss
- Chrome OS 72.0.3626.122

**Possible Solution**
Check the post content and make sure it isn't nothing but spaces (or something like that).

Start by tracing the discussion composer submission and post-content validation paths described in the report. Reproduce the single-space submission, then verify that whitespace-only content is rejected with a validation error and that normal post content remains accepted.

Written by the indexing model from the issue text.