Add isInitialized to nix::Value

Add a method to check if a value has been initialized. This helps avoid
segfaults when calling `type()`.
Useful in the context of the new C API.

Closes #10524

src/libexpr/value.hh

@@ -294,6 +294,11 @@ public:
         internalType = newType;
     }
 
+    inline bool isInitialized()
+    {
+        return internalType != 0;
+    }
+
     inline void mkInt(NixInt n)
     {
         finishValue(tInt, { .integer = n });

tests/unit/libexpr/value/value.cc

@@ -0,0 +1,24 @@
+#include "tests/libstore.hh"
+
+#include "value.hh"
+
+namespace nix {
+
+using namespace testing;
+
+TEST_F(LibStoreTest, unsetValue)
+{
+    Value unsetValue;
+    ASSERT_EQ(false, unsetValue.isInitialized());
+    ASSERT_EQ(tThunk, unsetValue.type(true));
+    ASSERT_DEATH(unsetValue.type(), "");
+}
+
+TEST_F(LibStoreTest, vInt)
+{
+    Value vInt;
+    vInt.mkInt(42);
+    ASSERT_EQ(true, vInt.isInitialized());
+}
+
+} // namespace nix