no not print empty keys in docker info

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

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
+}