Update-ZLocation $path should be mandatory

[theaquamarine]

Found while investigating #84 and #85, but probably not their cause: $path on Update-ZLocation isn't mandatory or provided with a default, so it's possible to run just Update-ZLocation. This makes it to the database and breaks ZLocation. The same is true of Add-ZWeight etc.

PS H:\> z
Weight Path
------ ----
     1 H:\
PS H:\> Update-ZLocation
PS H:\> z
Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
At C:\Program Files\WindowsPowerShell\Modules\ZLocation\1.2.0\ZLocation.Service.psm1:76 char:9
+         Write-Error $error[0]
+         ~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (:) [], ArgumentOutOfRangeException
    + FullyQualifiedErrorId : System.ArgumentOutOfRangeException

[theaquamarine]

Fixed in #87