fixed unused argument warning in read()

Author: mikir

compiler/extensions/cpp/freemarker/Union.cpp.ftl

@@ -701,7 +701,7 @@ void ${name}::read(<#rt>
 <#if isPackable && usedInPackedArray>
 
 void ${name}::read(<#rt>
-${name}::ZserioPackingContext&<#if uses_packing_context(fieldList)> context</#if><#rt>
+${name}::ZserioPackingContext&<#if fieldList?has_content> context</#if><#rt>
 , ::zserio::BitStreamReader&<#if fieldList?has_content> in</#if><#rt>
 <#if parameterArgs(true)?has_content>, ${parameterArgs(true)}</#if><#rt>
 , const allocator_type&<#if read_needs_allocator(compoundConstructorsData.fieldList)> allocator</#if>)