Remove the "shell" command from the completions, and allow empty lines

Even though you couldn't run a "shell" inside a "shell", the completion still
assumed you could :(

On the way - fixing error on empty lines: when you just press "enter", the shell should ignore it rather than try to run it as a command (and getting an error, obviously)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191605029

java/google/registry/tools/ShellCommand.java

@@ -102,6 +102,9 @@ public class ShellCommand implements Command {
     String line;
     while ((line = getLine()) != null) {
       String[] lineArgs = parseCommand(line);
+      if (lineArgs.length == 0) {
+        continue;
+      }
       try {
         runner.run(lineArgs);
       } catch (Exception e) {
@@ -242,5 +245,4 @@ public class ShellCommand implements Command {
           .collect(toImmutableList());
     }
   }
-
 }