fix: allow resource scope to be optional

[afiune]

Summary

The Lacework API indicates that the resource scope is optional but the code in the api go package
doesn't seem to support it.

This change will allow the api go package and the Lacework CLI to create Vulnerability Exceptions
with and without a Resource Scope.

How did you test this change?

Added tests and created two vulnerability exceptions, with and without Resource Scope.

Issue

https://lacework.atlassian.net/browse/ALLY-1017

[afiune]

The result of not having the resource scope is:

=== FAIL: api TestNewVulnerabilityExceptionNoResourceScope (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x133776b]

goroutine 2545 [running]:
testing.tRunner.func1.2({0x1461a80, 0x18381a0})
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1392 +0x39f
panic({0x1461a80, 0x18381a0})
	/usr/local/Cellar/go/1.18/libexec/src/runtime/panic.go:838 +0x207
github.com/lacework/go-sdk/api.(*VulnerabilityException).setResourceScope(0xc0002ab6c0, {0x0, 0x0})
	/Users/afiune/github/go-sdk/api/vulnerability_exceptions.go:237 +0x4b
github.com/lacework/go-sdk/api.NewVulnerabilityException({_, _}, {{0x14e1628, 0x18}, 0x0, 0x2, {0xc00028cb50, 0x1, 0x1}, {0x0, ...}, ...})
	/Users/afiune/github/go-sdk/api/vulnerability_exceptions.go:215 +0x3b3
github.com/lacework/go-sdk/api_test.TestNewVulnerabilityExceptionNoResourceScope(0xc00021b860?)
	/Users/afiune/github/go-sdk/api/vulnerability_exceptions_test.go:80 +0x1c5
testing.tRunner(0xc00021b860, 0x1532d00)
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1486 +0x35f

DONE 995 tests, 1 skipped, 1 failure in 6.219s
make: *** [test] Error 1