(chocolatey#310) Set-EnvironmentVariable: delete values properly

We noticed over the last few days that the previous version of this code
was *adding* blank values into the registry, which caused some issues
with how environment variables are applied to new processes.

Specifically, if a Machine-level variable is set, and a User-level
variable by the same name is set to a blank value, new processes do not
properly inherit the Machine-level value and it remains blank/unset.

This change ensures that if someone sets an environment variable to a
blank value, it will be properly *deleted* rather than set to a blank
value.

Additionally, a minor change has been made to Update-SessionEnvironment
to ensure that if there are somehow blank values in the registry, it can
sidestep those issues if it is called.

src/Chocolatey.PowerShell/Helpers/EnvironmentHelper.cs

@@ -166,7 +166,14 @@ public static void SetVariable(PSCmdlet cmdlet, string name, EnvironmentVariable
 
                 cmdlet.WriteDebug($"Registry type for {name} is/will be {registryType}");
 
-                registryKey.SetValue(name, value, registryType);
+                if (string.IsNullOrEmpty(value))
+                {
+                    registryKey.DeleteValue(name, throwOnMissingValue: false);
+                }
+                else
+                {
+                    registryKey.SetValue(name, value, registryType);
+                }
             }
 
             try
@@ -219,7 +226,10 @@ public static void UpdateSession(PSCmdlet cmdlet)
                 foreach (var name in GetVariableNames(scope))
                 {
                     var value = GetVariable(cmdlet, name, scope);
-                    SetVariable(cmdlet, name, EnvironmentVariableTarget.Process, value);
+                    if (!string.IsNullOrEmpty(value))
+                    {
+                        SetVariable(cmdlet, name, EnvironmentVariableTarget.Process, value);
+                    }
                 }
             }