Prevent request NullPointerException when there's a null value in a BasicNameValuePair parameter

Having a parameter like BasicNameValuePair("myParamName", null) throws a NullPointerException

```java
java.lang.NullPointerException at java.net.URLEncoder.encode(URLEncoder.java:92)
at com.foxykeep.datadroid.internal.network.NetworkConnectionImpl.execute(NetworkConnectionImpl.java:121)
at com.foxykeep.datadroid.network.NetworkConnection.execute(NetworkConnection.java:221)
```

I guess the same would happen for a null parameter name, but that's less likely to happen.

My approach was to not add the parameter at all, but maybe you'd prefer to handle the null value case by adding an empty string, which is safe.