[bug] Missing null check for certifyLegal with blobstore

[robert-cronin]

Describe the bug
A clear and concise description of what the bug is.
This bug is just a missing null check in the certifyLegal keyvalue code:

guac/pkg/assembler/backends/keyvalue/certifyLegal.go

Line 550 in 4db142d

out = append(out, legal)

Relevant disussion: #2168 (comment)

To Reproduce
Steps to reproduce the behavior:

1. Start GUAC up as usual with the blobstore backend
2. Ingest guac-main-data with guacone collect
3. Run the following query in the graphql IDE:

query CertifyLegalQ1 {
    CertifyLegal(certifyLegalSpec: {subject: {package: {name: "log4j-core"} } }) {
        id
    }
}

Expected behavior
Return either empty for no result or that found result, i.e.:

Actual
The actual query will fail for each element because there is no null check on the legalIfMatch result, e.g. for the above query

    {
      "message": "the requested element is null which the schema does not allow",
      "path": [
        "CertifyLegal",
        21
      ]
    },

GUAC version
Are you using a released version of GUAC or one build from a commit in the
repository?
v0.9.1

Additional context
N/A

[robert-cronin]

I'll add a fix for this in another PR