lldb: don't issue command when there's no env variables to set, fix Bug 391897

Summary:
LLDB becomes stricter about the `settings set target.env-vars`
command. Now it doens't allow empty arguments. This causes a warning
when start debugging session in KDevelop.

https://bugs.kde.org/show_bug.cgi?id=391897

Test Plan:
- Created a new project with an empty environment variable profile
- Start debugging with lldb
- No warnings

Reviewers: apol, #kdevelop

Reviewed By: apol, #kdevelop

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D11524

(cherry picked from commit 3fd20921)