Update path rendering criteria to require source/target node availability

[napakalas]

Revisiting the current rendering logic: examples from keast-4 and keast-8 show inconsistencies due to missing T13 and L6 nodes in the human flatmap.

• keast-4 is rendered based on minimum-degree node completeness, despite missing source nodes.
• keast-8 is not rendered due to a missing minimum-degree node, but also lacks target nodes.

Proposal: prioritise availability of phenotype-defined source and target nodes over minimum-degree node checks when determining rendering eligibility.