Kernel: Allow passing null pointer to delete

The C++ standard says that it's legal to call the `delete` operator with
a null pointer argument, in which case it should be a no-op. I
encountered this issue when running a kernel that's compiled with Clang.
I assume this fact was used for some kind of optimization.

Kernel/KString.cpp

@@ -59,6 +59,8 @@ OwnPtr<KString> KString::try_clone() const
 
 void KString::operator delete(void* string)
 {
+    if (!string)
+        return;
     size_t allocation_size = sizeof(KString) + (sizeof(char) * static_cast<KString*>(string)->m_length) + sizeof(char);
     kfree_sized(string, allocation_size);
 }