DB: Set correct default value for numeric types

Set 0 as default value for columns with numeric data type instead of the
empty string ''. Otherwise the database complains about an invalid
default value for this column.

To reproduce put the following in your ````appinfo/database.xml````:

````
<field>
        <name>modified</name>
        <type>decimal</type>
        <default/>
        <notnull>true</notnull>
        <length>15</length>
</field>
````

See owncloud/mozilla_sync#14