A2DP: Check if offload capability is set

Check if offload capability is valid. If the property is not
defined get property will return empty string. Check for empty
string and set offload cap to null. This is a fix for mulitcast
flag getting disabled eventhough offload cap property was not
defined.

Change-Id: Ia647289a0d3b58d3c0b41f1cc8872ed5021f2844