AK+Everywhere: Remove the null state of DeprecatedString

This commit removes DeprecatedString's "null" state, and replaces all
its users with one of the following:
- A normal, empty DeprecatedString
- Optional<DeprecatedString>

Note that null states of DeprecatedFlyString/StringView/etc are *not*
affected by this commit. However, DeprecatedString::empty() is now
considered equal to a null StringView.

Userland/Libraries/LibCore/Account.h

@@ -41,7 +41,7 @@ public:
     ErrorOr<void> login() const;
 
     DeprecatedString username() const { return m_username; }
-    DeprecatedString password_hash() const { return m_password_hash; }
+    DeprecatedString password_hash() const { return m_password_hash.value_or(DeprecatedString::empty()); }
 
     // Setters only affect in-memory copy of password.
     // You must call sync to apply changes.
@@ -56,9 +56,9 @@ public:
     void set_extra_gids(Vector<gid_t> extra_gids) { m_extra_gids = move(extra_gids); }
     void delete_password();
 
-    // A null password means that this account was missing from /etc/shadow.
+    // A nonexistent password means that this account was missing from /etc/shadow.
     // It's considered to have a password in that case, and authentication will always fail.
-    bool has_password() const { return !m_password_hash.is_empty() || m_password_hash.is_null(); }
+    bool has_password() const { return !m_password_hash.has_value() || !m_password_hash->is_empty(); }
 
     uid_t uid() const { return m_uid; }
     gid_t gid() const { return m_gid; }
@@ -82,7 +82,7 @@ private:
 
     DeprecatedString m_username;
 
-    DeprecatedString m_password_hash;
+    Optional<DeprecatedString> m_password_hash;
     uid_t m_uid { 0 };
     gid_t m_gid { 0 };
     DeprecatedString m_gecos;