[QAST::Block] only create hashes when .symbol sets

When the getter variant of .symbol is used, it now returns an
undefined value, rather than autovivify a hash for it.

src/QAST/Block.nqp

@@ -27,9 +27,11 @@ class QAST::Block is QAST::Node {
     }
 
     method symbol($name, *%attrs) {
-        %!symbol{$name} := %!symbol{$name} // {};
-        for %attrs {
-            %!symbol{$name}{$_.key} := $_.value;
+        if %attrs {
+            %!symbol{$name} := %!symbol{$name} // {};
+            for %attrs {
+                %!symbol{$name}{$_.key} := $_.value;
+            }
         }
         %!symbol{$name}
     }

t/qast/qast.t

@@ -741,3 +741,9 @@ is_qast(
     my %hash := $block.symbol('slug');
     ok(%hash<food> eq 'cheeseburger', 'QAST::Block.symbol existing attributes don\'t disappear');
 }
+
+{
+    my $block := QAST::Block.new();
+    my $missing := $block.symbol('sabre-toothed tiger');
+    ok(!pir::defined($missing), 'QAST::Block.symbol on a nonexistent key returns an undefined value');
+}