dropEmptyInitThenConcatStringSep: Check that we don't drop...

... initial empty strings.

The tests pass, which is encouraging.

src/libexpr/symbol-table.hh

@@ -41,6 +41,11 @@ public:
     }
 
     friend std::ostream & operator <<(std::ostream & os, const SymbolStr & symbol);
+
+    bool empty() const
+    {
+        return s->empty();
+    }
 };
 
 /**

src/libutil/util.hh

@@ -40,6 +40,13 @@ template<class C>
 std::string dropEmptyInitThenConcatStringsSep(const std::string_view sep, const C & ss)
 {
     size_t size = 0;
+
+    for (auto & i : ss) {
+        // Make sure we don't rely on the empty item ignoring behavior
+        assert(!i.empty());
+        break;
+    }
+
     // need a cast to string_view since this is also called with Symbols
     for (const auto & s : ss) size += sep.size() + std::string_view(s).size();
     std::string s;