Fix web service crash if set flag with invalid request parameter

[luyade]

What type of PR is this?

• bug

What problem(s) does this PR solve?

Issue(s) number: fix #5506

Description:

Fix web service crash if the request parameter is invalid. In this issue, if the config value is not string, the item.second.asString() in the source code will lead an exception.

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

ex. Fixed the bug .....

[yixinglu]

LGTM