fix: handle unset serial number (#4440)

Signed-off-by: Terri Oda <terri.oda@intel.comm>

cve_bin_tool/output_engine/__init__.py

@@ -803,7 +803,7 @@ def output_cves(self, outfile, output_type="console"):
                 self.vex_type,
                 self.all_cve_data,
                 self.vex_product_info["revision_reason"],
-                self.vex_product_info["sbom_serial_number"],
+                self.vex_product_info.get("sbom_serial_number", ""),
                 logger=self.logger,
             )
             vexgen.generate_vex()