fix add functions to agent(#4937)

[davidxwwang]

Why are these changes needed?

fix bug: in class AgentBuilder~build() add function（list_of_functions to agent，the return agent name is ‘None’，and the app crashed

Related issue number

#4937

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[davidxwwang]

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[ekzhu]

Please run pre-commit --all-files to fix format