Reverse empty simple values as an empty names sequence rather than empty name

This way we get:

config.import.foo =

Rather than:

config.import.foo = {}

1 files changed, 2 insertions, 0 deletions

diff --git a/build2/variable b/build2/variable
index 1248bd3..fb44bb4 100644
--- a/build2/variable
+++ b/build2/variable
@@ -476,6 +476,7 @@ namespace build2
     static void append (value&, bool); // OR.
     static name reverse (bool x) {return name (x ? "true" : "false", false);}
     static int compare (bool, bool);
+    static bool empty (bool) {return false;}
 
     static const char* const type_name;
     static const build2::value_type value_type;
@@ -491,6 +492,7 @@ namespace build2
     static void append (value&, uint64_t); // ADD.
     static name reverse (uint64_t x) {return name (to_string (x), false);}
     static int compare (uint64_t, uint64_t);
+    static bool empty (bool) {return false;}
 
     static const char* const type_name;
     static const build2::value_type value_type;