[IZPACK-1180] ConfigurationInstallerListener: Nested <entry> remains empty if it didn't exist before

Fixes https://jira.codehaus.org/browse/IZPACK-1180:

If a property file is created by ConfigurationListener, or nested <entry> doesn't still exist before applyiong configuration actions, the nested entries are left empty and their value attribute isn't applied at all.
Example
```xml
<configurable type="options" keepOldKeys="true" keepOldValues="true" tofile="${INSTALL_PATH}/db.properties">
        <entry key="driver" value="org.firebirdsql.jdbc.FBDriver" />
        <entry key="url" value="jdbc:firebirdsql:${db.host}/${db.port}:##user.dir#/" />
        <entry key="file" value="${database.scriptsfolder}/${db.name}.fdb" />
        <entry key="user" value="${db.user}" />
        <entry key="pass" value="${db.password}" />
</configurable>
```
if db.properties doesn't exist the ConfigurationInstallerListener leaves it in this state:
```
driver=
url=
file=
user=
pass=
```