Just replace the "invalid" timestamp value with something else. E.g.:
index=tenable
| eval PatchPubDate=if(patchPubDate=-1, "unknown", strftime(patchPubDate,"%Y-%m-%d"))
| stats by riskFactor,severity.id,pluginID,description,solution,pluginText,PatchPubDate,ip
or if you still need it to be a timestamp, then:
index=tenable
| eval PatchPubDate=if(patchPubDate=-1, "1970-01-01", strftime(patchPubDate,"%Y-%m-%d"))
| stats by riskFactor,severity.id,pluginID,description,solution,pluginText,PatchPubDate,ip
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