Guard empty float-or-percent input

[BenJule]

Summary

• guard both float-or-percent parsing paths before calling std::string::back()
• preserve the existing percentage parsing behavior
• let empty input continue into the existing exception-based validation path

Problem

change_opt_value() called str.back() for coFloatOrPercent and coFloatsOrPercents without first checking whether the input string was empty.

Calling std::string::back() on an empty string is undefined behavior and may crash before the surrounding exception handler can report the invalid value.

Testing

• verified both parsing paths contain the empty-string guard
• ran git diff --check
• ran a targeted GUI.cpp syntax check using an existing compile database

No local full build was performed.

[tonghao-bbl]

LGTM @lanewei120

[lanewei120]

thanks @BenJule