Don't add empty input lines to command history

This change fixes PowerShell/vscode-powershell#618 which reports that
empty input lines are being added to command history in the integrated
console.  This change updates the command input logic to skip sending
empty lines to the command executor.

Author: daviwil

src/PowerShellEditorServices/Console/ConsoleService.cs

@@ -314,16 +314,19 @@ await this.consoleReadLine.ReadCommandLine(
                 {
                     Console.Write(Environment.NewLine);
 
-                    var unusedTask =
-                        this.powerShellContext
-                            .ExecuteScriptString(
-                                commandString,
-                                false,
-                                true,
-                                true)
-                            .ConfigureAwait(false);
-
-                    break;
+                    if (!string.IsNullOrWhiteSpace(commandString))
+                    {
+                        var unusedTask =
+                            this.powerShellContext
+                                .ExecuteScriptString(
+                                    commandString,
+                                    false,
+                                    true,
+                                    true)
+                                .ConfigureAwait(false);
+
+                        break;
+                    }
                 }
             }
             while (!cancellationToken.IsCancellationRequested);