Use a zero fixed-price if useMarketBasedPrice=true

The CLI and apitest cases always pass "0", but java & python clients
might pass an empty string. This change avoids number formatting
& scaling problems when clients pass an empty string in the price
parameter.

core/src/main/java/bisq/core/api/CoreApi.java

@@ -206,7 +206,7 @@ public void createAndPlaceOffer(String currencyCode,
                                     Consumer<Offer> resultHandler) {
         coreOffersService.createAndPlaceOffer(currencyCode,
                 directionAsString,
-                price,
+                useMarketBasedPrice ? "0" : price,
                 useMarketBasedPrice,
                 marketPriceMargin,
                 amountAsLong,