Failed dataref lookups are transmitted as 0.0 without a guard

Describe the bug
If XPLMFindDataRef fails it returns NULL, and XPLMGetDataf then returns 0.0 (XPLMDataAccess.h:342-344: "the dataref value or 0.0 if the dataref is NULL or the plugin is disabled"). The eight-value vector is built and sent with no validity check between construction and baton->send (xplane_plugin/src/pilotdatasync-xp11.cpp:423-440), so a failed lookup transmits 0.0. That is indistinguishable from a real zero reading, and downstream an altitude of 0 is labeled TAXI.

Expected behavior
DataRef handles are checked for NULL after lookup, and values from a failed lookup are not transmitted, or are sent as something iMotions and the logger can recognize as missing.

Additional context
The plugin checks std::isnan for display (xplane_plugin/src/pilotdatasync-xp11.cpp:267-271), but a failed read never produces NaN, so that check never triggers. The same gap exists on the button path at :144-150. This appears to be an unfinished part of #63.

Found while writing the setup documentation in #138 / #178.

Start in xplane_plugin/src/pilotdatasync-xp11.cpp at lines 423-440 and trace the dataref lookup, eight-value vector construction, and baton->send call. Also inspect the button path at lines 144-150 and the display NaN check at lines 267-271. Done means failed lookups are guarded and their values are not transmitted as indistinguishable 0.0 readings.