AK: Make HashMap::get(Key) return an Optional<Value>.

This allows HashMap::get() to be used for value types that cannot be default
constructed (e.g NonnullOwnPtr.)

Kernel/KParams.cpp

@@ -29,7 +29,7 @@ KParams::KParams(const String& cmdline)
 
 String KParams::get(const String& key) const
 {
-    return m_params.get(key);
+    return m_params.get(key).value_or({});
 }
 
 bool KParams::has(const String& key) const