Allow editoffer clients to pass empty string for price parameter

The client may have passed an empty string for the price parameter,
if only enabling or disabling the offer.  If so, validate with new
price = old price.

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

@@ -68,7 +68,12 @@ class EditOfferValidator {
                        int newEnable,
                        EditOfferRequest.EditType editType) {
         this.currentlyOpenOffer = currentlyOpenOffer;
-        this.newPrice = newPrice;
+        // The client may have passed an empty string for the price parameter
+        // if only enabling or disabling the offer.  If so, validate with new
+        // price = old price.
+        this.newPrice = editType.equals(ACTIVATION_STATE_ONLY)
+                ? currentlyOpenOffer.getOffer().getPrice().toString()
+                : newPrice;
         // The client cannot determine what offer.isUseMarketBasedPrice should be
         // when editType = ACTIVATION_STATE_ONLY.  Override newIsUseMarketBasedPrice
         // param for the ACTIVATION_STATE_ONLY case.