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.

Tests/AK/TestCircularDeque.cpp

@@ -41,7 +41,7 @@ TEST_CASE(enqueue_begin_being_moved_from)
 
     DeprecatedString str { "test" };
     strings.enqueue_begin(move(str));
-    EXPECT(str.is_null());
+    EXPECT(str.is_empty());
 }
 
 TEST_CASE(deque_end)

Tests/AK/TestDeprecatedString.cpp

@@ -14,11 +14,9 @@
 
 TEST_CASE(construct_empty)
 {
-    EXPECT(DeprecatedString().is_null());
     EXPECT(DeprecatedString().is_empty());
-    EXPECT(!DeprecatedString().characters());
+    EXPECT(DeprecatedString().characters() != nullptr);
 
-    EXPECT(!DeprecatedString("").is_null());
     EXPECT(DeprecatedString("").is_empty());
     EXPECT(DeprecatedString("").characters() != nullptr);
 
@@ -29,7 +27,6 @@ TEST_CASE(construct_contents)
 {
     DeprecatedString test_string = "ABCDEF";
     EXPECT(!test_string.is_empty());
-    EXPECT(!test_string.is_null());
     EXPECT_EQ(test_string.length(), 6u);
     EXPECT_EQ(test_string.length(), strlen(test_string.characters()));
     EXPECT(test_string.characters() != nullptr);
@@ -42,7 +39,7 @@ TEST_CASE(construct_contents)
 
 TEST_CASE(equal)
 {
-    EXPECT_NE(DeprecatedString::empty(), DeprecatedString {});
+    EXPECT_EQ(DeprecatedString::empty(), DeprecatedString {});
 }
 
 TEST_CASE(compare)
@@ -116,7 +113,7 @@ TEST_CASE(move_string)
     auto test_string_copy = test_string;
     auto test_string_move = move(test_string_copy);
     EXPECT_EQ(test_string, test_string_move);
-    EXPECT(test_string_copy.is_null());
+    EXPECT(test_string_copy.is_empty());
 }
 
 TEST_CASE(repeated)
@@ -253,7 +250,6 @@ TEST_CASE(builder_zero_initial_capacity)
     StringBuilder builder(0);
     builder.append(""sv);
     auto built = builder.to_deprecated_string();
-    EXPECT_EQ(built.is_null(), false);
     EXPECT_EQ(built.length(), 0u);
 }
 

Tests/AK/TestHashMap.cpp

@@ -49,7 +49,7 @@ TEST_CASE(range_loop)
 
     int loop_counter = 0;
     for (auto& it : number_to_string) {
-        EXPECT_EQ(it.value.is_null(), false);
+        EXPECT_EQ(it.value.is_empty(), false);
         ++loop_counter;
     }
     EXPECT_EQ(loop_counter, 3);

Tests/AK/TestHashTable.cpp

@@ -64,7 +64,7 @@ TEST_CASE(range_loop)
 
     int loop_counter = 0;
     for (auto& it : strings) {
-        EXPECT_EQ(it.is_null(), false);
+        EXPECT_EQ(it.is_empty(), false);
         ++loop_counter;
     }
     EXPECT_EQ(loop_counter, 3);

Tests/AK/TestJSON.cpp

@@ -60,7 +60,6 @@ TEST_CASE(json_empty_string)
 {
     auto json = JsonValue::from_string("\"\""sv).value();
     EXPECT_EQ(json.type(), JsonValue::Type::String);
-    EXPECT_EQ(json.as_string().is_null(), false);
     EXPECT_EQ(json.as_string().is_empty(), true);
 }
 
@@ -68,7 +67,6 @@ TEST_CASE(json_string)
 {
     auto json = JsonValue::from_string("\"A\""sv).value();
     EXPECT_EQ(json.type(), JsonValue::Type::String);
-    EXPECT_EQ(json.as_string().is_null(), false);
     EXPECT_EQ(json.as_string().length(), size_t { 1 });
     EXPECT_EQ(json.as_string() == "A", true);
 }
@@ -77,7 +75,6 @@ TEST_CASE(json_utf8_character)
 {
     auto json = JsonValue::from_string("\"\\u0041\""sv).value();
     EXPECT_EQ(json.type(), JsonValue::Type::String);
-    EXPECT_EQ(json.as_string().is_null(), false);
     EXPECT_EQ(json.as_string().length(), size_t { 1 });
     EXPECT_EQ(json.as_string() == "A", true);
 }
@@ -92,7 +89,6 @@ TEST_CASE(json_utf8_multibyte)
 
     auto& json = json_or_error.value();
     EXPECT_EQ(json.type(), JsonValue::Type::String);
-    EXPECT_EQ(json.as_string().is_null(), false);
     EXPECT_EQ(json.as_string().length(), size_t { 2 });
     EXPECT_EQ(json.as_string() == "š", true);
     EXPECT_EQ(json.as_string() == "\xc5\xa1", true);

Tests/AK/TestVector.cpp

@@ -44,14 +44,12 @@ TEST_CASE(strings)
 
     int loop_counter = 0;
     for (DeprecatedString const& string : strings) {
-        EXPECT(!string.is_null());
         EXPECT(!string.is_empty());
         ++loop_counter;
     }
 
     loop_counter = 0;
     for (auto& string : (const_cast<Vector<DeprecatedString> const&>(strings))) {
-        EXPECT(!string.is_null());
         EXPECT(!string.is_empty());
         ++loop_counter;
     }