The Contracts with Approval standard report type was designed to support the legacy Contract Approval function and not the Standard Workflow and Approval Feature. If you are not using the legacy Contract Approval Function, it is expected that no records will show up in the report.
If you create a custom report type with Process Instance as the primary object and Process Instance Node as the related object, you can create approval history reports with various combinations of fields that enable you to view a detailed history of executed and in-progress approval processes and their individual steps. Please visit this link for more details.
Also, please be aware of the limitation currently posted in ideas exchange.
Approval History as a relatable object in Custom Report Types (CRT)
Other helpful reference:
Examples of Approval History Reports
Additional exceptions apply to approval history data that is available only via SOQL queries of certain objects. See ProcessInstance, ProcessInstanceNode, ProcessInstanceStep, and ProcessInstanceWorkitem in the Object Reference for Salesforce and Force.com.
Note: With the Spring ’15 release*, we will retire the legacy Contract Approvals function and it will become read-only.