lib/srdb1: schema - don't set default value for text/blob columns of topos

- not allowed by newer versions of mysql

lib/srdb1/schema/topos_d.xml

@@ -128,21 +128,21 @@
    <column id="a_rr">
         <name>a_rr</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>Record route - A side</description>
 	</column>
 
     <column id="b_rr">
         <name>b_rr</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>Record route - B side</description>
     </column>
 
     <column id="s_rr">
         <name>s_rr</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>Record route - S side</description>
     </column>
 

lib/srdb1/schema/topos_t.xml

@@ -88,7 +88,7 @@
     <column id="x_via">
         <name>x_via</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>Via stack - A or B side</description>
     </column>
 
@@ -104,21 +104,21 @@
     <column id="x_rr">
         <name>x_rr</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>RR stack - incoming A or B side</description>
 	</column>
 
     <column id="y_rr">
         <name>y_rr</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>RR stack - the other A or B side</description>
 	</column>
 
    <column id="s_rr">
         <name>s_rr</name>
         <type>largetext</type>
-        <default/>
+        <null/>
         <description>RR stack - the S side</description>
 	</column>