Omit 'inherited from' heading if there are no extended methods

If the collector does not resolve inheritance the generator still writes the 'inherited from' heading into the output, although there are no methods listed underneath it.

This configuration:
```xml
<project name="apidox" source="${basedir}/src" workdir="${basedir}/build/apidox/xml">
    <!--  Additional configuration for the collecting process (parse of php code, generation of xml data) -->
    <collector publiconly="true" backend="parser">
        <include mask="*.php"/>
        <exclude mask=""/>
        <!--  How to handle inheritance -->
        <inheritance resolve="false"/>
    </collector>

    <!--  Configuration of generation process -->
    <generator output="${basedir}/apidocs">
        <enrich base="${basedir}/build"/>

        <build engine="html" enabled="true" output="html">
            <template dir="${phpDox.home}/templates/html"/>
            <file extension="xhtml"/>
        </build>

    </generator>
</project>
```

leads to this output for a class that extends another class:
```html
<h2 id="methods">Methods</h2>
<div class="styled">
    <h4>public</h4>
    <ul>
        <li id="foo">
            <a title="B" href="../classes/B/foo.xhtml">foo()</a>
            — Method foo in Class B
        </li>
    </ul>
    <h3>
        Inherited from
        <a title="" href="../classes/.xhtml"></a>
    </h3>
</div>
```

The h3 element shouldn't be there.