style: add ValidateNotNullOrEmpty to build.ps1 -Task parameter (#41)

Matches the canonical template (PowerShellModuleTemplate#23). Without
the validator, `./build.ps1 -Task ''` and `./build.ps1 -Task $null` slip
past parameter binding and produce confusing downstream errors from
psake. The validator surfaces the misuse immediately.

Author: tablackburn

build.ps1

@@ -40,6 +40,7 @@ param(
                 @()
             }
         })]
+    [ValidateNotNullOrEmpty()]
     [string[]]$Task = 'default',
 
     # Bootstrap dependencies