fix bug with empty boot command.

common/bootcommand/boot_command_ast.go

@@ -75,11 +75,14 @@ func (s expressionSequence) Validate() (errs []error) {
 // GenerateExpressionSequence generates a sequence of expressions from the
 // given command. This is the primary entry point to the boot command parser.
 func GenerateExpressionSequence(command string) (expressionSequence, error) {
+	seq := expressionSequence{}
+	if command == "" {
+		return seq, nil
+	}
 	got, err := ParseReader("", strings.NewReader(command))
 	if err != nil {
 		return nil, err
 	}
-	seq := expressionSequence{}
 	for _, exp := range got.([]interface{}) {
 		seq = append(seq, exp.(expression))
 	}

common/bootcommand/boot_command_ast_test.go

@@ -138,3 +138,9 @@ func Test_validation(t *testing.T) {
 		}
 	}
 }
+
+func Test_empty(t *testing.T) {
+	exp, err := GenerateExpressionSequence("")
+	assert.NoError(t, err, "should have parsed an empty input okay.")
+	assert.Len(t, exp, 0)
+}