When no quotas exist "list --include-workspace-size" returns size "-1"

[VMois]

Originated in reanahub/reana-db#178 (comment)

When quotas are disabled, there could be a situation when returned workspace size is "-1". This result might be confusing for users. In theory, after the quota cronjob is run, the size should not equal to "-1".

Related code:

reana-workflow-controller/reana_workflow_controller/rest/workflows.py

Lines 321 to 324 in 9d6b34d

	empty_disk_usage = {
	"human_readable": "",
	"raw": -1,
	}