fix: warn user when filament serial input sanitizes to empty string

[BenJule]

Summary

• Show a warning message when the user enters a filament serial number that gets sanitized (trimmed/filtered) to an empty string, preventing silent data loss

Changes

• src/slic3r/GUI/CreatePresetsDialog.cpp: Add validation that warns the user when serial input becomes empty after sanitization

Test plan

• Enter a filament serial number consisting only of whitespace or invalid characters
• Verify a warning is shown instead of silently saving an empty serial
• Verify normal serial numbers still save correctly