no not print empty keys in docker info

Signed-off-by: Victor Vieux <victorvieux@gmail.com>

pkg/ioutils/fmt.go

@@ -0,0 +1,14 @@
+package ioutils
+
+import (
+	"fmt"
+	"io"
+)
+
+// FprintfIfNotEmpty prints the string value if it's not empty
+func FprintfIfNotEmpty(w io.Writer, format, value string) (int, error) {
+	if value != "" {
+		return fmt.Fprintf(w, format, value)
+	}
+	return 0, nil
+}

pkg/ioutils/fmt_test.go

@@ -0,0 +1,17 @@
+package ioutils
+
+import "testing"
+
+func TestFprintfIfNotEmpty(t *testing.T) {
+	wc := NewWriteCounter(&NopWriter{})
+	n, _ := FprintfIfNotEmpty(wc, "foo%s", "")
+
+	if wc.Count != 0 || n != 0 {
+		t.Errorf("Wrong count: %v vs. %v vs. 0", wc.Count, n)
+	}
+
+	n, _ = FprintfIfNotEmpty(wc, "foo%s", "bar")
+	if wc.Count != 6 || n != 6 {
+		t.Errorf("Wrong count: %v vs. %v vs. 6", wc.Count, n)
+	}
+}