Disabling UseInProcessClient via cake.config fails, when there are pre existing packages inside the tools directory

### What You Are Seeing?
There is an ArgumentNullException being thrown, when i run cake 0.25 with older versions of cake addins and tools in the tools folder

### What is Expected?
No Exception should be thrown and cake should be able to update its dependencies according to its requirements.

### What version of Cake are you using?
0.25, after upgrade from 0.24

### Are you running on a 32 or 64 bit system?
x64

### What environment are you running on? Windows? Linux? Mac?
Windows 10 Pro 1709

### Are you running on a CI Server? If so, which one?
local build

### How Did You Get This To Happen? (Steps to Reproduce)
Was running a local build in a closed source project after upgrading from cake 0.24. I can provide further details if required.

[StackTrace](https://files.gitter.im/cake-build/cake/bUmt/image.png)

Cleaning my tools folder besides my pinned nuget.config solved the issue. @devlead Wanted me to create this issue for possible integration tests and further improvements

Edit: here is the relevant part of my cake.config

```
[Nuget]
Source=https://packages.nuget.org/api/v2;https://somePrivateNugetFeed.json
UseInProcessClient=false
```

Start with the cake.config settings for UseInProcessClient and the existing packages in the tools directory, then use the linked stack trace and the pinned nuget.config to reproduce the upgrade from Cake 0.24 to 0.25. Done means the build throws no ArgumentNullException and updates dependencies according to its requirements, with coverage for pre-existing tools.

Written by the indexing model from the issue text.