Issue #16828: Fix error incorrectly raised by bz2.compress('').

Patch by Martin Packman.
diff --git a/Modules/bz2module.c b/Modules/bz2module.c
index 4795965..2e99df9 100644
--- a/Modules/bz2module.c
+++ b/Modules/bz2module.c
@@ -1979,7 +1979,7 @@
         return NULL;
     }
 
-    action = BZ_RUN;
+    action = input_left > 0 ? BZ_RUN : BZ_FINISH;
 
     for (;;) {
         char *saved_next_out;