fix: dropSpecialCharacters false also should return prefix [ref:#686]

[NepipenkoIgor]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/JsDaddy/ngx-mask/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

DropSpecialCharacters false also should return prefix

Issue Number: #686

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No