repo fork: check that --org is not the empty string

As it is already being done for --remote-name, except in this case
the default is the empty string.

pkg/cmd/repo/fork/fork.go

@@ -83,6 +83,10 @@ Additional 'git clone' flags can be passed in by listing them after '--'.`,
 				opts.GitArgs = args[1:]
 			}
 
+			if cmd.Flags().Changed("org") && opts.Organization == "" {
+				return &cmdutil.FlagError{Err: errors.New("--org cannot be blank")}
+			}
+
 			if opts.RemoteName == "" {
 				return &cmdutil.FlagError{Err: errors.New("--remote-name cannot be blank")}
 			}

pkg/cmd/repo/fork/fork_test.go

@@ -120,6 +120,12 @@ func TestNewCmdFork(t *testing.T) {
 				Organization: "batmanshome",
 			},
 		},
+		{
+			name:    "empty org",
+			cli:     " --org=''",
+			wantErr: true,
+			errMsg:  "--org cannot be blank",
+		},
 		{
 			name:    "git flags in wrong place",
 			cli:     "--depth 1 OWNER/REPO",