Fixed problem when one of the trend values is None.

Regression test in test suites.

bin/weewx/tags.py

@@ -440,7 +440,10 @@ class TrendObj(object):
                 # F_to_C(68F - 50F). We want the former, not the latter.
                 now_vtc  = self.converter.convert(now_vt)
                 then_vtc = self.converter.convert(then_vt)
-                trend = now_vtc - then_vtc
+                if now_vtc.value is None or then_vtc.value is None:
+                    trend = ValueTuple(None, now_vtc.unit, now_vtc.group)
+                else:
+                    trend = now_vtc - then_vtc
             
         # Return the results as a ValueHelper. Use the formatting and labeling
         # options from the current time record. The user can always override

bin/weewx/test/expected/index.html

@@ -91,6 +91,10 @@
       <td>Outside Temperature trend with explicit time_delta (3600 seconds)</td>
       <td>4.4&#176;F</td>
     </tr>
+    <tr>
+      <td>Outside Temperature trend where previous value is known to be None (9000 seconds)</td>
+      <td>   N/A</td>
+    </tr>
     <tr>
       <td>Trend with nonsense type</td>
       <td>?'foobar'?</td>

bin/weewx/test/expected/metric/index.html

@@ -91,6 +91,10 @@
       <td>Outside Temperature trend with explicit time_delta (3600 seconds)</td>
       <td>2.4&#176;C</td>
     </tr>
+    <tr>
+      <td>Outside Temperature trend where previous value is known to be None (9000 seconds)</td>
+      <td>   N/A</td>
+    </tr>
     <tr>
       <td>Trend with nonsense type</td>
       <td>?'foobar'?</td>

bin/weewx/test/test_skins/StandardTest/index.html.tmpl

@@ -98,6 +98,11 @@
       <td>Outside Temperature trend with explicit time_delta (3600 seconds)</td>
       <td>$trend($time_delta=3600).outTemp</td>
     </tr>
+    <tr>
+      ## This test uses a previous value known to be None:
+      <td>Outside Temperature trend where previous value is known to be None (9000 seconds)</td>
+      <td>$trend($time_delta=9000).outTemp</td>
+    </tr>
     <tr>
       <td>Trend with nonsense type</td>
       <td>$trend.foobar</td>