Avoid populating Select2 input with existing option

[Toshakins]

Resolves #622.

After searching through the Select2 docs, I've found that pre-population can be avoided by adding a multiple attribute: https://select2.org/getting-started/basic-usage#multi-select-boxes-pillbox

I confirm the update fixes the linked issue.

Again, huge thanks to @aminalaee for his work and research on the clues to resolve the issue.

Still, I am not sure about the next stuff:

• Should I add some tests, and if yes, where and how?
• Can my change break somebody's setup?

[aminalaee]

I've tested it in different cases and seems to work in all cases. and also this Select2 widget is only used in ARRAY fields so the impact is limited.

[aminalaee]

LGTM, thanks for the PR.