fix(voice): leave out an unreported altitude instead of saying 0 feet

select_nearest_aircraft passes altitudeM: null for an airborne contact
without a barometric altitude (military rows from adsb.lol can lack
alt_baro). The speech builder coerced it with Number(), and Number(null)
is 0, so the reply said "..., 0 feet." and the voice card showed
"Altitude 0 feet". Read the altitude only when it is a finite number,
so an unknown one is left out as the builder already intends.

src/voice/speech.js

@@ -74,9 +74,9 @@ function listPhrase(items) {
 }
 
 function feetRounded(meters) {
-  const value = Number(meters);
-  return Number.isFinite(value)
-    ? Math.round((value * 3.28084) / 100) * 100
+  // A missing altitude is unknown, not sea level: Number(null) is 0.
+  return Number.isFinite(meters)
+    ? Math.round((meters * 3.28084) / 100) * 100
     : null;
 }