Ensure GetNetworkCredential() returns null if PSCredential has null or empty user name (#4697)

* GetNetworkCredential() returns null if PSCredential has empty user name

* Fix test

test/powershell/engine/Basic/Credential.Tests.ps1

@@ -0,0 +1,6 @@
+Describe "Credential tests" -Tags "CI" {
+    It "Explicit cast for an empty credential returns null" {
+         # We should explicitly check that the expression returns $null
+         [PSCredential]::Empty.GetNetworkCredential() | Should Be $null
+    }
+}