Fix dijkstraVia bad alloc on 4.0.2

[cvvergara]

Fixes #3110: pgr_dijkstraVia crash when via vertex not in graph
pgr_dijkstraVia threw a bad_alloc error when a via vertex did not exist in the graph. The fix adds an empty-path check before accessing paths.front()
When a via vertex is missing, the function now returns an empty result set instead of crashing.

File changes summary:

• CMakeLists.txt: Version bump to 4.0.2
• include/dijkstra/dijkstra.hpp: Return empty Path when paths is empty
• pgtap/dijkstra/dijkstraVia/edge_cases/issue-3110.pg: New test covering via vertices that exist, don't exist, and are missing
  between valid vertices
  • Back-port note: The min_version('4.0.1') guard in the test skips on older versions where the bug still exists.
• NEWS.md, doc/src/release_notes.rst: Release notes for the fix

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