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
Add DB tests for default numeric

Add a table with a numeric column and an empty default directive.
changed type to decimal
treat numeric as an alias for decimal
Merge pull request #5789 from ogasser/db_fix_default_for_numeric

DB: Set correct default value for numeric types