[oneDPL][ranges][merge] support size limit for output; + check n==0

include/oneapi/dpl/pstl/algorithm_ranges_impl.h

@@ -448,6 +448,9 @@ auto
 __pattern_merge(_Tag __tag, _ExecutionPolicy&& __exec, _R1&& __r1, _R2&& __r2, _OutRange&& __out_r, _Comp __comp,
                 _Proj1 __proj1, _Proj2 __proj2)
 {
+    using __return_type = std::ranges::merge_result<std::ranges::borrowed_iterator_t<_R1>, std::ranges::borrowed_iterator_t<_R2>,
+        std::ranges::borrowed_iterator_t<_OutRange>>;
+
     auto __comp_2 = [__comp, __proj1, __proj2](auto&& __val1, auto&& __val2) { return std::invoke(__comp,
         std::invoke(__proj1, std::forward<decltype(__val1)>(__val1)), std::invoke(__proj2,
         std::forward<decltype(__val2)>(__val2)));};
@@ -464,10 +467,10 @@ __pattern_merge(_Tag __tag, _ExecutionPolicy&& __exec, _R1&& __r1, _R2&& __r2, _
     auto __it_2 = std::ranges::begin(__r2);
     auto __it_out = std::ranges::begin(__out_r);
 
-    auto __res = __pattern_merge_2(__tag, std::forward<_ExecutionPolicy>(__exec), __it_2, __n_2, __it_1, __n_1, __it_out, __n_out, __comp_2);
+    if(__n_out == 0)
+        return __return_type{__it_1, __it_2, __it_out};
 
-    using __return_type = std::ranges::merge_result<std::ranges::borrowed_iterator_t<_R1>, std::ranges::borrowed_iterator_t<_R2>,
-        std::ranges::borrowed_iterator_t<_OutRange>>;
+    auto __res = __pattern_merge_2(__tag, std::forward<_ExecutionPolicy>(__exec), __it_2, __n_2, __it_1, __n_1, __it_out, __n_out, __comp_2);
 
     return __return_type{__res.second, __res.first, __it_out + __n_out};
 }