fix #493: strip whitespace from current_query and selection

[tconbeer]

Closes #493

What are the key elements of this solution?
This PR strips whitespace from editor.current_query, editor.previous_query, and app._validate_selection, so that the previous query is executed when the current query is only whitespace. The previous behavior was confusing (see issue).

Why did you design your solution this way? Did you assess any alternatives? Are there tradeoffs?
The database doesn't care about whitespace, so it's better to strip it than add new logic with isspace, etc.

Does this PR require a change to Harlequin's docs?

• No.
• Yes, and I have opened a PR at tconbeer/harlequin-web.
• Yes; I haven't opened a PR, but the gist of the change is: ...

Did you add or update tests for this change?

• Yes.
• No, I believe tests aren't necessary.
• No, I need help with testing this change.

Please complete the following checklist:

• I have added an entry to CHANGELOG.md, under the [Unreleased] section heading. That entry references the issue closed by this PR.
• I acknowledge Harlequin's MIT license. I do not own my contribution.

[ml31415]

Big thanks for the quick fix!