Add empty string parameter to start command

Otherwise `start` will confuse the first parameter as the title of
a new command prompt if the parameter contains whitespace. That is
because the command to be run will be:

    start "abc def"

which opens a new command prompt window with the title "abc def".
With the added empty string we force the start command to interpret
the passed parameter as the file / command:

    start "" "abc def"

which will be interpreted like `""` is the title and the rest is
the file or command to start.

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**NOTE:** this wouldn't be necessary if the start script in msys
was defined differently; that is, if it had the empty string
already incorporated in the script (/usr/bin/start), like so:

```diff
-cmd //c start "${@//&/^&}"
+cmd //c start "" "${@//&/^&}"
```

Notice however that this would make it impossible to use start
setting a different title, so it's probably best to leave it as is.

More info: http://sourceforge.net/p/msys2/tickets/14/

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The change `${(z)open_cmd}` is necessary to force zsh to split the
variable by the spaces and interpret it as separate words.

More info: http://zsh.sourceforge.net/FAQ/zshfaq03.html#l17