device/bluetooth: Fix copy constructor of BluetoothServiceAttributeValueBlueZ

The copy constructor was trying to call DeepCopy() against a nullptr. This adds
a check if the type is NULLTYPE and return base::Value() if so.

BUG=b:30963813
TEST=call the copy constructor with a NULLTYPE instance to see if it crashes

Review-Url: https://codereview.chromium.org/2284713002
Cr-Commit-Position: refs/heads/master@{#414746}

device/bluetooth/bluez/bluetooth_service_attribute_value_bluez.cc

@@ -33,6 +33,11 @@ BluetoothServiceAttributeValueBlueZ::BluetoothServiceAttributeValueBlueZ(
   this->type_ = attribute.type_;
   this->size_ = attribute.size_;
 
+  if (attribute.type_ == NULLTYPE) {
+    this->value_ = base::Value::CreateNullValue();
+    return;
+  }
+
   if (attribute.type_ != SEQUENCE) {
     this->value_ = base::WrapUnique(attribute.value_->DeepCopy());
     return;