fix(cctv): keep missing numbers unknown instead of reading them as zero

Number() reads null, blank strings and booleans as 0 (or 1), and
/api/cctv/sources sends an unknown number as JSON null. A source-pack
camera that left out its pose therefore reached the browser as zeros
clamped to each field's floor (north, pitch -2, FOV 20, range 120 m,
mount 6 m, ground 0 m) instead of taking the defaults, and a null or
blank position put the camera at 0,0.

toFiniteNumber (server) and safeNumber (browser) now return the fallback
for absent, blank and boolean values, as validateBox already does for
Recent Imagery. The same conversion fixes the per-pack paths: a Tallinn
row with a null heading no longer reads as a high-confidence north-facing
camera, a Caltrans camera with a blank elevation keeps its 150 m ground
prior, and TfL or Catalonia cameras with a null or blank coordinate are
dropped instead of kept at 0,0 or on the zero meridian.

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+- Keep missing CCTV numbers unknown instead of reading them as zero. `Number`
+  turns null, blank strings and booleans into 0 or 1, and the camera source
+  list sends an unknown number as JSON null, so a source-pack camera that left
+  out its pose reached the browser as zeros clamped to each field's floor —
+  facing north, pitch −2°, FOV 20°, range 120 m, mount 6 m and ground 0 m —
+  instead of the defaults (id-hash heading, −17°, 74°, 700 m, 24 m and the
+  city's ground); a null or blank position placed the camera at 0,0. The
+  server's `toFiniteNumber` and the browser's `safeNumber` now treat absent,
+  blank and boolean values as missing. The same conversion also stops a
+  Tallinn row with a null heading reading as a confident north-facing camera,
+  a Caltrans camera with a blank elevation losing its 150 m ground prior, and
+  a TfL or Catalonia camera with a null or blank coordinate being kept at 0,0
+  or on the zero meridian.
+
 - Cyclone advisories no longer go dark while NHC publishes an advisory ahead
   of its nominal time: advisory and position times up to 90 minutes ahead of
   the server clock are accepted (previously 5 minutes), so the whole feed is