Traverse: Do not short-circuit traverse_all() if stmts is nullptr

Since commit 0a813a53c7,
zeek::detail::stmts is set to a nullptr when before it was usually
an empty list of statements. This caused traverse_all() to be
short-circuited unless global statements were available.

Fixes #4323

src/Traverse.cc

@@ -2,27 +2,70 @@
 
 #include "zeek/Traverse.h"
 
+#include "zeek/Func.h"
 #include "zeek/Scope.h"
-#include "zeek/Stmt.h"
 #include "zeek/input.h"
 
+#include "zeek/3rdparty/doctest.h"
+
 namespace zeek::detail {
 
 TraversalCode traverse_all(TraversalCallback* cb) {
     if ( ! global_scope() )
         return TC_CONTINUE;
 
-    if ( ! stmts )
-        // May be null when parsing fails.
-        return TC_CONTINUE;
 
     cb->current_scope = global_scope();
 
     TraversalCode tc = global_scope()->Traverse(cb);
-
     HANDLE_TC_STMT_PRE(tc);
-    tc = stmts->Traverse(cb);
+
+    if ( stmts )
+        // May be null when parsing fails.
+        tc = stmts->Traverse(cb);
+
     HANDLE_TC_STMT_POST(tc);
 }
 
 } // namespace zeek::detail
+
+
+TEST_SUITE_BEGIN("traverser");
+
+namespace {
+// Helper classes for tests below.
+using namespace zeek::detail;
+
+class SaveRestoreStmts {
+public:
+    SaveRestoreStmts() : orig(zeek::detail::stmts) {}
+    ~SaveRestoreStmts() { zeek::detail::stmts = orig; }
+
+    Stmt* orig;
+};
+
+class ZeekInitFinder : public TraversalCallback {
+public:
+    TraversalCode PreFunction(const zeek::Func* f) override {
+        if ( f->GetName() == "zeek_init" )
+            zeek_init_found = true;
+
+        return TC_CONTINUE;
+    }
+
+    bool zeek_init_found = false;
+};
+
+} // namespace
+
+TEST_CASE("traverse_all") {
+    SUBCASE("ensure zeek_init() is found if stmts == nullptr") {
+        SaveRestoreStmts save_restore_stmts;
+        zeek::detail::stmts = nullptr; // force stmts to be a nullptr
+        ZeekInitFinder cb;
+        traverse_all(&cb);
+        CHECK(cb.zeek_init_found);
+    }
+}
+
+TEST_SUITE_END();