mod_sasl2_fast: Improve handling when SASL profile unexpectedly lacks CB
This fixes a traceback reported by riau, but likely does not solve the
underlying cause, whatever that is.
This module provides two [metrics][doc:statistics]:
`lua_heap_bytes`
: Bytes of memory as reported by `collectgarbage("count")`{.lua}
`lua_info`
: Provides the current Lua version as a label
``` openmetrics
# HELP lua_info Lua runtime version
# UNIT lua_info
# TYPE lua_info gauge
lua_info{version="Lua 5.4"} 1
# HELP lua_heap_bytes Memory used by objects under control of the Lua
garbage collector
# UNIT lua_heap_bytes bytes
# TYPE lua_heap_bytes gauge
lua_heap_bytes 8613218
```