Fix bug that caused empty files invalid

Now EndOfFileRule will not fail on empty files neither on files
consisting of a single newline.

It will however fail on files consisting of nothing but newlines, if
SingleNewLine option is active.

linter/eof.go

@@ -30,7 +30,13 @@ func NewEndOfFileRule(config Config) Linter {
 
 // Lint implements the Lint interface
 func (rule EndOfFileRule) Lint(b []byte) (valid bool, fix []byte) {
-	if rule.SingleNewLine {
+
+	// for empty files
+	if len(b) < 1 {
+		return true, nil
+	}
+
+	if rule.SingleNewLine && len(b) > 1 {
 		valid = regexp.MustCompile(`[^\n]\n\z`).Match(b)
 	} else {
 		valid = regexp.MustCompile(`\n\z`).Match(b)

linter/eof_test.go

@@ -1,6 +1,7 @@
 package linter
 
 import (
+	"fmt"
 	"testing"
 )
 
@@ -52,6 +53,38 @@ func TestEOFLint_TextWithoutNewLine(t *testing.T) {
 	}
 }
 
+func TestEOFLint_EmptyString(t *testing.T) {
+
+	// empty files are valid
+	got, _ := NewEndOfFileRule(autofixTestConf).Lint([]byte(""))
+
+	if got != true {
+		t.Errorf("NewEndOfFileRule(autofixTestConf).Lint(emptyFileText):\n\tExpected %v, got %v", true, got)
+	}
+}
+
+func TestEOFLint_StringWithOneNewline(t *testing.T) {
+	// files with a single newline char are also valid
+	got, _ := NewEndOfFileRule(autofixTestConf).Lint([]byte(fmt.Sprintf("\n")))
+
+	if got != true {
+		t.Errorf("NewEndOfFileRule(autofixTestConf).Lint(emptyFileText):\n\tExpected %v, got %v", true, got)
+	}
+}
+
+func TestEOFLint_StringWithTwoNewlines(t *testing.T) {
+	// files with a two newlines should be reduced to one newline if singleNewLineRule is set
+	got, fixed := NewEndOfFileRule(autofixTestConf).Lint([]byte(fmt.Sprintf("\n\n")))
+
+	if string(fixed) != string(fmt.Sprintf("\n")) {
+		t.Errorf("NewEndOfFileRule(autofixTestConf).Lint(textWithoutNewLine): autofix did not work\n\tExpected:\n%q\n\tGot:\n%q", fmt.Sprintf("\n\n"), string(fixed))
+	}
+
+	if got != false {
+		t.Errorf("NewEndOfFileRule(autofixTestConf).Lint(emptyFileText):\n\tExpected %v, got %v", false, got)
+	}
+}
+
 func TestEOFLint_NotTextFile(t *testing.T) {
 	// the 0xFFFD UTF-8 control character should be ignored, because the Lint method
 	// does not check if the input is a valid Text file or not 'IsText' check fail