Fix broken local serve due to azure logging middleware failure

[jjgriff93]

Fixes #28

When docker compose is used (for serve-local), it sets any unset variables as empty strings in the container environment. So, as would be typical for local scenarios not using application insights, when unset, the logging initialisation fails as it only catches for the env var being None, when it also needs to catch for empty strings.

Have amended with a check before the logging is initialised, which will skip if the APPLICATIONINSIGHTS_CONNECTION_STRING is None or "", logging a warning message to the console if so