res_config_odbc: set empty extended field as a single whitespace

If Realtime @ variable value is NULL or empty or contains only whitespaces
then when we try to retrieve it using PJSIP_ENDPOINT we get WARNING
pjsip_endpoint_function_read: Unknown property @my_var for PJSIP endpoint.
And the variable is missing in the result of CLI pjsip show endpoint.

This patch keeps empty sorcery extended field.

ASTERISK-28341 #close

Change-Id: I221fccc04cbfa2be17ce971f64ae0e74e465eea0