schema: allow null in active_watchers reason

(cherry picked from commit 2eccc2342b929fea96bd006f386948f483e62d06)

src/lib/srdb1/schema/pr_active_watchers.xml

@@ -153,6 +153,7 @@
         <name>reason</name>
         <type>string</type>
         <size>&user_len;</size>
+        <null/>
         <description>Reason</description>
     </column>