If the audit log was deleted then there would be an entry similar to this:
| TIMESTAMP | HOST | PORT | SERVICE_NAME | CONNECTION_ID | CLIENT_HOST | CLIENT_IP | CLIENT_PID | CLIENT_PORT | USER_NAME | APPLICATION_USER_NAME | AUDIT_POLICY_NAME | EVENT_STATUS | EVENT_LEVEL | EVENT_ACTION | SCHEMA_NAME | OBJECT_NAME | PRIVILEGE_NAME | ROLE_NAME | GRANTEE | GRANTABLE | FILE_NAME | SECTION | KEY | PREV_VALUE | VALUE | STATEMENT_STRING | COMMENT |
| 2015-10-28 02:58:29.0 | xxxxx | 30703 | indexserver | 207655 | xxxx.corp.sap | 10.42.134.177 | 14728 | 60521 | ADMIN | I028297 | MandatoryAuditPolicy | SUCCESSFUL | CRITICAL | CLEAR AUDIT LOG | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | ? | alter system clear audit log until '2015-10-09 06:26:08.0' | ? |
The audit log table cannot be "just empty" once the auditing had been activated at one point in time as operations managing the auditing are also logged (at least in the SP 10 version I'm using).
You haven't commented so far on where the audit stream is written to on your system.