improve TEST_REQUIRES backward compat line

Author: rehsack

Makefile.PL

@@ -82,10 +82,7 @@ sub WriteMakefile1
     die "License not specified"    if ( !exists( $params{LICENSE} ) );
     $params{TEST_REQUIRES}
       and $eumm_version < 6.6303
-      and $params{BUILD_REQUIRES} =
-      $params{BUILD_REQUIRES}
-      ? { %{ $params{BUILD_REQUIRES} }, %{ delete $params{TEST_REQUIRES} } }
-      : delete $params{TEST_REQUIRES};
+      and $params{BUILD_REQUIRES} = { %{ $params{BUILD_REQUIRES} || {} }, %{ delete $params{TEST_REQUIRES} } };
     #EUMM 6.5502 has problems with BUILD_REQUIRES
     $params{BUILD_REQUIRES}
       and $eumm_version < 6.5503