Fix optional primitive decoder.

Author: marciorasf-ab

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/RustGenerator.java

@@ -894,6 +894,13 @@ private static void generatePrimitiveOptionalDecoder(
             formatFunctionName(name),
             rustPrimitiveType);
 
+        if (fieldToken.version() > 0)
+        {
+            indent(sb, level + 1, "if self.acting_version() < %d {\n", fieldToken.version());
+            indent(sb, level + 2, "return None;\n");
+            indent(sb, level + 1, "}\n\n");
+        }
+
         indent(sb, level + 1, "let value = self.get_buf().get_%s_at(self.%s);\n",
             rustPrimitiveType,
             getBufOffset(fieldToken));