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