Allow usage-graph to not depend on pipelineArtifactsSize

What does this MR do?

It allows usage-graph component to not use/depend on pipelineArtifactsSize, the component will be used in !68866 (merged) as part of https://gitlab.com/groups/gitlab-org/-/epics/6264, where project statistics does not have pipelineArtifactsSize property.

Related to #334888

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)