fix(mesh): null-check getMeshNode in handleFromRadio

[DatanoiseTV]

NodeDB::getMeshNode returns nullptr legitimately under heap pressure
(see NodeDB::isFull and updateFrom early-out paths). The condition at
MeshService.cpp:97 dereferenced the result directly to read has_user,
so every received decoded packet from a new node could crash on a
device near pool exhaustion. Restructure with an init-statement so the
node is fetched once and the rest of the predicate short-circuits on
null.

Split out from #10424 per @thebentern's request — single-concern PR.

Build verification

pio run -e t-deck-tft succeeds, no new warnings.

Attestations

• I have tested that my proposed changes behave as described — review/static-analysis only, not on-air.
• On-hardware testing requested from community: build-verified t-deck-tft only.

[Copilot]

Pull request overview

Fixes a potential null-pointer dereference in MeshService::handleFromRadio() when processing decoded packets from nodes that may not be present in the NodeDB (e.g., under low-memory / full-DB conditions), preventing a crash while deciding whether to proactively send NodeInfo.

Changes:

• Refactors the NodeInfo-send predicate to fetch the sender node once using a C++17 init-statement.
• Adds a null check (senderNode && ...) before reading senderNode->has_user, ensuring safe short-circuiting when getMeshNode() returns nullptr.