Fix empty string encoding in AssetCreateTransaction (#3)

Empty strings for url, unitName, and assetName were being encoded
as "au": "", "un": "", "an": "" in MessagePack instead of being
omitted. Algorand's canonical encoding requires empty/default values
to be omitted entirely, causing signature verification failures.

Added !isEmpty checks to properly omit empty strings from encoding.

Author: 0xLeif

Sources/Algorand/AssetTransaction.swift

@@ -128,13 +128,13 @@ public struct AssetCreateTransaction: Transaction {
             apar["df"] = .bool(true)
         }
 
-        if let unitName = assetParams.unitName {
+        if let unitName = assetParams.unitName, !unitName.isEmpty {
             apar["un"] = .string(unitName)
         }
-        if let assetName = assetParams.assetName {
+        if let assetName = assetParams.assetName, !assetName.isEmpty {
             apar["an"] = .string(assetName)
         }
-        if let url = assetParams.url {
+        if let url = assetParams.url, !url.isEmpty {
             apar["au"] = .string(url)
         }
         if let metadataHash = assetParams.metadataHash {