22274: Bugfix graphs existence check

22274: Bugfix graphs existence check

Earlier code always assigns `self.graphs[managed_graph]` and
`self.graphs[storage_graph]`. When the graphs aren't available (e.g.,
because the user doesn't have Prometheus credentials), the values are
None. This would cause us to run `len(None)` and crash with a
TypeError.

Simplify by checking the truthiness of graph data and then calculating
the rest of what we need.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>