Add nil check on events dataset pagination

[linkous8]

Description

Adds nil check to fsoc optimize events related datasets during pagination logic. Prevents following stack trace:

✓ Platform API call (POST /monitoring/v1/query/execute)
   ⨯ Execution of events query encountered errors. Returned data may not be complete!
   ⨯ General Error: Event data error (logged with TraceId: [3063c1ccd79d60f5d55db6b8006b196c])
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x38 pc=0x102e96b34]

goroutine 1 [running]:
github.com/cisco-open/fsoc/cmd/optimize.listEvents.func1(0x140001ec300?, {0x1400032b030?, 0x0?, 0x7?})
	github.com/cisco-open/fsoc/cmd/optimize/events.go:220 +0x8e4
github.com/spf13/cobra.(*Command).execute(0x140001ec300, {0x140000b2030, 0x7, 0x7})
	github.com/spf13/cobra@v1.7.0/command.go:940 +0x5c8
github.com/spf13/cobra.(*Command).ExecuteC(0x1036ef4c0)
	github.com/spf13/cobra@v1.7.0/command.go:1068 +0x35c
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v1.7.0/command.go:992
github.com/spf13/cobra.(*Command).ExecuteContext(...)
	github.com/spf13/cobra@v1.7.0/command.go:985
github.com/cisco-open/fsoc/cmd.Execute(...)
	github.com/cisco-open/fsoc/cmd/root.go:87
main.realMain()
	github.com/cisco-open/fsoc/main.go:36 +0x114
main.main()
	github.com/cisco-open/fsoc/main.go:28 +0x1c

Type of Change

• Bug Fix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass