show "?" instead of 0.000 amounts.

src/page.h

@@ -165,10 +165,10 @@ struct tx_details
             {"pub_key"           , tx_pk_str},
             {"tx_fee"            , fee_str},
             {"tx_fee_short"      , fee_short_str},
-            {"sum_inputs"        , fmt::format("{:0.6f}", XMR_AMOUNT(xmr_inputs))},
-            {"sum_outputs"       , fmt::format("{:0.6f}", XMR_AMOUNT(xmr_outputs))},
-            {"sum_inputs_short"  , fmt::format("{:0.3f}", XMR_AMOUNT(xmr_inputs))},
-            {"sum_outputs_short" , fmt::format("{:0.3f}", XMR_AMOUNT(xmr_outputs))},
+            {"sum_inputs"        , xmr_amount_to_str(xmr_inputs , "{:0.6f}")},
+            {"sum_outputs"       , xmr_amount_to_str(xmr_outputs, "{:0.6f}")},
+            {"sum_inputs_short"  , xmr_amount_to_str(xmr_inputs , "{:0.3f}")},
+            {"sum_outputs_short" , xmr_amount_to_str(xmr_outputs, "{:0.3f}")},
             {"no_inputs"         , static_cast<uint64_t>(input_key_imgs.size())},
             {"no_outputs"        , static_cast<uint64_t>(output_pub_keys.size())},
             {"mixin"             , mixin_str},

src/tools.h

@@ -253,7 +253,14 @@ static
 string
 xmr_amount_to_str(const uint64_t& xmr_amount, string _format="{:0.12f}")
 {
-    return fmt::format(_format, XMR_AMOUNT(xmr_amount));
+    string amount_str = "?";
+
+    if (xmr_amount > 0)
+    {
+        amount_str = fmt::format(_format, XMR_AMOUNT(xmr_amount));
+    }
+
+    return amount_str;
 }
 
 bool