Do not create the admin menu item if the component has no admin files

**Problem**
Joomla! create a useless menu item for backend, when I create a simple component (with only a site part) that have no backend.

**How to test this pull**
Make a simple component, and try install it. Create folder `com_hello`, and put there two files `hello.php` (empty) and `hello.xml`

`hello.xml` content for test the component without backed:
```xml
<?xml version="1.0" encoding="utf-8"?>
<extension type="component" version="3.3" method="upgrade">
   <name>com_hello</name>
   <author>Your Name</author>
   <authorEmail>your.email@example.com</authorEmail>
   <authorUrl>example.com</authorUrl>
   <creationDate>September 2014</creationDate>
   <license>GNU/GPL</license>
   <version>1.0.0</version>
   <description>Simple Hello component</description>

   <files folder="site">
      <file>hello.php</file>
   </files>

</extension>
```

`hello.xml` content for test the component with backed:

```xml
<?xml version="1.0" encoding="utf-8"?>
<extension type="component" version="3.3" method="upgrade">
   <name>com_hello</name>
   <author>Your Name</author>
   <authorEmail>your.email@example.com</authorEmail>
   <authorUrl>example.com</authorUrl>
   <creationDate>September 2014</creationDate>
   <license>GNU/GPL</license>
   <version>1.0.0</version>
   <description>Simple Hello component</description>

   <files folder="site">
      <file>hello.php</file>
   </files>

   <administration>
        <files />
    </administration>

</extension>
```

Pack this to archive and try install.
After installation, check the admin menu Components->com-hello. It should not exist when test with the first XML, and exist for a second.