fix crashes inserting nil into array

Attempts to fix the following crash:
I think the real (safe) solution might be to not return `nil` in `-[QSObject objectWithString:]`, but that would probably break a lot of of other code

```
Version:         ß72 (3945)

OS Version:      Mac OS X 10.8.2 (12C60)

Crashed Thread:  9  Dispatch queue: com.apple.root.low-priority

Exception Type:  EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000

Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]'
abort() called
terminate called throwing an exception

Application Specific Backtrace 1:
0   CoreFoundation                      0x00007fff8c3ea0a6 __exceptionPreprocess + 198
1   libobjc.A.dylib                     0x00007fff88bc43f0 objc_exception_throw + 43
2   CoreFoundation                      0x00007fff8c39c6b7 -[__NSPlaceholderArray initWithObjects:count:] + 119
3   CoreFoundation                      0x00007fff8c3bf882 +[NSArray arrayWithObject:] + 50
4   Keychain Plugin                     0x000000010575e038 objc_unretainedPointer + 4694

       0x10575b000 -        0x105760fff +com.blacktree.Quicksilver.QSKeychainPlugIn (1.1.0 - 13E) <45AE4F1E-5797-3E5B-9B02-BEB5DD46BEBA> /Users/USER/Library/Application Support/Quicksilver/*/Keychain Plugin
```