Add UTIL_IsStringEmpty helper, replace strlen()==0 pattern

[jkivilin]

Summary

• Add inline UTIL_IsStringEmpty() helper in util.h that checks str[0] == '\0' instead of walking the string with strlen()
• Replace strlen(in_name) == 0 in bot_chat.cpp with UTIL_IsStringEmpty(in_name)
• Add 4 unit tests for the new helper

Closes #84