[lldb][ConstString] Prevent GetString from constructing a std::string with a nullptr (#95175)

This patch prevents passing a `nullptr` to the `std::string` constructor
in `GetString`. This prevents UB arising from calling `GetString` on a
default-constructed `ConstString`.

Author: Michael137

lldb/include/lldb/Utility/ConstString.h

@@ -199,7 +199,9 @@ class ConstString {
   }
 
   /// Get the string value as a std::string
-  std::string GetString() const { return std::string(m_string, GetLength()); }
+  std::string GetString() const {
+    return std::string(AsCString(""), GetLength());
+  }
 
   /// Get the string value as a C string.
   ///

lldb/unittests/Utility/ConstStringTest.cpp

@@ -88,6 +88,7 @@ TEST(ConstStringTest, NullAndEmptyStates) {
   EXPECT_TRUE(!null);
   EXPECT_TRUE(null.IsEmpty());
   EXPECT_TRUE(null.IsNull());
+  EXPECT_TRUE(null.GetString().empty());
 }
 
 TEST(ConstStringTest, CompareConstString) {