translateModelFMU returns empty string on failure

This makes it possible to script the result of translateModelFMU, to
compare the result with empty string (or the expected string).

Author: sjoelund

Compiler/Script/CevalScriptBackend.mo

@@ -1317,11 +1317,17 @@ algorithm
         (cache,ret_val,st_1) := buildModelFMU(cache, env, className, st, str1, str2, filenameprefix, true);
       then (cache,ret_val,st_1);
 
+    case (cache,env,"translateModelFMU", _,st,_)
+      then (cache,Values.STRING(""),st);
+
     case (cache,env,"buildModelFMU", Values.CODE(Absyn.C_TYPENAME(className))::Values.STRING(str1)::Values.STRING(str2)::Values.STRING(filenameprefix)::Values.ARRAY(valueLst=cvars)::_,st,_)
       algorithm
         (cache,ret_val,st_1) := buildModelFMU(cache, env, className, st, str1, str2, filenameprefix, true, list(ValuesUtil.extractValueString(vv) for vv in cvars));
       then (cache,ret_val,st_1);
 
+    case (cache,env,"buildModelFMU", _,st,_)
+      then (cache,Values.STRING(""),st);
+
     case (cache,env,"translateModelXML",{Values.CODE(Absyn.C_TYPENAME(className)),Values.STRING(filenameprefix)},st,_)
       equation
         filenameprefix = Util.stringReplaceChar(filenameprefix,".","_");