hack/make.ps1: fix Validate-PkgImports

In Go 1.10.1, the Deps for pkg\tarsum\tarsum_test.go are empty ([]) and
the PowerShell script ends up setting its import list to a string value
of False instead of an empty array. This can be remedied by forcing the
result to be an array, by concatenating to an empty array (@() + ...)

Signed-off-by: John Stephens <johnstep@docker.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit ec3f9230d70506c536a24e844da0f0b3af9b43f6)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/make.ps1

@@ -251,10 +251,10 @@ Function Validate-PkgImports($headCommit, $upstreamCommit) {
         if ($LASTEXITCODE -ne 0) { Throw "Failed go list for dependencies on $file" }
         $imports = $imports -Replace "\[" -Replace "\]", "" -Split(" ") | Sort-Object | Get-Unique
         # Filter out what we are looking for
-        $imports = $imports -NotMatch "^github.com/docker/docker/pkg/" `
-                            -NotMatch "^github.com/docker/docker/vendor" `
-                            -Match "^github.com/docker/docker" `
-                            -Replace "`n", ""
+        $imports = @() + $imports -NotMatch "^github.com/docker/docker/pkg/" `
+                                  -NotMatch "^github.com/docker/docker/vendor" `
+                                  -Match "^github.com/docker/docker" `
+                                  -Replace "`n", ""
         $imports | % { $badFiles+="$file imports $_`n" }
     }
     if ($badFiles.Length -eq 0) {