mitchellh/packer — pull request #1509 by kamazee (2014-09-18 04:16:52+00:00)
Title: Fix detecting guest additions URL

When `Default Guest Additions ISO` system property exists with empty value, packer treats that empty value as a URL. The issue has been spotted on Arch with Virtualbox 4.3.16.

Here is the output of `vboxmanage list systemproperties` packer built from master choked with:

```
API version: 4_3
Minimum guest RAM size: 4 Megabytes
Maximum guest RAM size: 2097152 Megabytes
Minimum video RAM size: 1 Megabytes
Maximum video RAM size: 256 Megabytes
Maximum guest monitor count: 64
Minimum guest CPU count: 1
Maximum guest CPU count: 32
Virtual disk limit (info): 2199022206976 Bytes
Maximum Serial Port count: 2
Maximum Parallel Port count: 2
Maximum Boot Position: 4
Maximum PIIX3 Network Adapter count: 8
Maximum ICH9 Network Adapter count: 36
Maximum PIIX3 IDE Controllers: 1
Maximum ICH9 IDE Controllers: 1
Maximum IDE Port count: 2
Maximum Devices per IDE Port: 2
Maximum PIIX3 SATA Controllers: 1
Maximum ICH9 SATA Controllers: 8
Maximum SATA Port count: 30
Maximum Devices per SATA Port: 1
Maximum PIIX3 SCSI Controllers: 1
Maximum ICH9 SCSI Controllers: 8
Maximum SCSI Port count: 16
Maximum Devices per SCSI Port: 1
Maximum SAS PIIX3 Controllers: 1
Maximum SAS ICH9 Controllers: 8
Maximum SAS Port count: 8
Maximum Devices per SAS Port: 1
Maximum PIIX3 Floppy Controllers:1
Maximum ICH9 Floppy Controllers: 1
Maximum Floppy Port count: 1
Maximum Devices per Floppy Port: 2
Default machine folder: /home/alex/VirtualBox VMs
Exclusive HW virtualization use: on
Default hard disk format: VDI
VRDE auth library: VBoxAuth
Webservice auth. library: VBoxAuth
Remote desktop ExtPack: Oracle VM VirtualBox Extension Pack
Log history count: 3
Default frontend:
Autostart database path:
Default Guest Additions ISO:
```