Add validation for vulnerability identifiers

What does this MR do and why?

Fixes #373760

Ensures that at least one identifier is provided when using the vulnerabilityCreate GraphQL mutation.

Prior to the validation, when no identifiers were provided, an Internal Server Error would occur.

@bwill already provided most of the code in this issue comment: #373760 (comment 1563579009)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Follow steps from issue: #373760