fix: handle empty files gracefully instead of throwing errors

- Remove empty data validation in CountBytes method
- Empty files now return zero statistics instead of errors
- Update tests to reflect new behavior
- Add comprehensive empty data handling tests

Fixes issue where empty files would cause program to crash with
'input data cannot be empty' error during directory scanning.

Author: 100gle

count.go

@@ -117,11 +117,8 @@ func isChinese(r rune) bool {
 //   - Minimal function call overhead
 //   - Local variables to reduce struct field access overhead
 //
-// Returns an error if the input data is empty.
+// Empty data is handled gracefully and returns zero counts for all statistics.
 func (c *Counter) CountBytes(data []byte) error {
-	if len(data) == 0 {
-		return NewInvalidInputError("input data cannot be empty")
-	}
 
 	// Use local variables to minimize struct field access overhead
 	lines := 0

count_test.go

@@ -190,12 +190,12 @@ func TestCounter_CountBytes_EdgeCases(t *testing.T) {
 		{
 			name:    "Empty byte slice",
 			input:   []byte{},
-			wantErr: true,
+			wantErr: false,
 		},
 		{
 			name:    "Nil byte slice",
 			input:   nil,
-			wantErr: true,
+			wantErr: false,
 		},
 		{
 			name:    "Single newline",
@@ -226,6 +226,40 @@ func TestCounter_CountBytes_EdgeCases(t *testing.T) {
 	}
 }
 
+func TestCounter_CountBytes_EmptyData(t *testing.T) {
+	tests := []struct {
+		name  string
+		input []byte
+	}{
+		{
+			name:  "Empty byte slice",
+			input: []byte{},
+		},
+		{
+			name:  "Nil byte slice",
+			input: nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tc := wordcounter.NewCounter()
+			err := tc.CountBytes(tt.input)
+
+			if err != nil {
+				t.Errorf("Counter.CountBytes() error = %v, expected no error for empty data", err)
+				return
+			}
+
+			stats := tc.GetStats()
+			if stats.Lines != 0 || stats.ChineseChars != 0 || stats.NonChineseChars != 0 || stats.TotalChars != 0 {
+				t.Errorf("Counter.CountBytes() for empty data, expected all zero stats, got: Lines=%d, ChineseChars=%d, NonChineseChars=%d, TotalChars=%d",
+					stats.Lines, stats.ChineseChars, stats.NonChineseChars, stats.TotalChars)
+			}
+		})
+	}
+}
+
 func TestCounter_LargeText(t *testing.T) {
 	// Test with large text to ensure performance
 	largeText := strings.Repeat("这是一个测试文本。", 10000) // 10,000 repetitions