feat: check for ret <= 0 in atclient_monitor_read

[JeremyTubongbanua]

- What I did

• feat: pkam refactor and reconnection examples #281 fixed connection.c's atclient_connection_send's issue with ret == 0 as an error,
• this PR also adds that same error handling in atclient_monitor_read.
• this comment is a good explanation of why we should check for the mbedtls_ssl_read(...) == 0 case.

- How I did it

- How to verify it

• at_talk works

- Description for the changelog
feat: check for ret <= 0 in atclient_monitor_read