Allow stringifying sample with unknown pressure

lib/xmet/sounding.py

@@ -54,9 +54,10 @@ class SoundingSample(DatabaseTable):
         self.wind_speed:  float = None
 
     def __str__(self):
-        parts = [
-            ("%.2fmb" % self.pressure)
-        ]
+        parts = list()
+
+        if self.pressure is not None:
+            parts.append("%.2fmb" % self.pressure)
 
         if self.height is not None:
             parts.append("%.1fm" % self.height)