add a LexPad test to cover a corner case

git-svn-id: https://svn.parrot.org/parrot/trunk@49623 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

t/pmc/lexpad.t

@@ -19,10 +19,11 @@ Tests the LexPad PMC.
 
 .sub main :main
     .include 'test_more.pir'
-    plan(8)
+    plan(9)
 
     new_test()
     test_keyed()
+    test_keyed_empty()
     test_iter()
 .end
 
@@ -63,6 +64,15 @@ eh:
 end:
 .end
 
+.sub test_keyed_empty
+    .local pmc info, pad, lex
+    info = new ['LexInfo']
+    pad = new ['LexPad'], info
+    lex = pad['nosuchlex']
+    $I0 = isnull lex
+    ok($I0, 'get an item from empty lexpad gives null')
+.end
+
 .sub 'test_iter'
 
     .local pmc str1,str2,str3