String property deserializes null as "null" for JsonTypeInfo.As.EXISTING_PROPERTY

[jonc2]

Describe the bug
When using JsonTypeInfo.As.EXISTING_PROPERTY to deserialize String properties, a null value is being deserialized as "null"

Version information
2.12.2

To Reproduce
If you have a way to reproduce this with:

public class ExistingPropertyShouldBeNullTest {

    @JsonTypeInfo(use = Id.NAME, include = As.EXISTING_PROPERTY, visible = true, property = "type", defaultImpl = DefaultShape.class)
    @JsonSubTypes({@Type(value = Square.class, name = "square")})
    static abstract class Shape {

        public String type;

        public String getType() { return this.type; }

        public void setType(String type) { this.type = type; }
    }

    static class Square extends Shape {}

    static class DefaultShape extends Shape {}

    private final ObjectMapper MAPPER = new ObjectMapper();

    @Test
    public void testDeserializationWithValidType() throws Exception {
        Shape deserShape = MAPPER.readValue("{\"type\":\"square\"}", Shape.class);
        assertEquals("square", deserShape.getType());
    }

    @Test
    public void testDeserializationWithInvalidType() throws Exception {
        Shape deserShape = MAPPER.readValue("{\"type\":\"invalid\"}", Shape.class);
        assertEquals("invalid", deserShape.getType());
    }

    @Test
    public void testDeserializationNull() throws Exception {
        Shape deserShape = MAPPER.readValue("{\"type\":null}", Shape.class);
        assertNull(deserShape.getType()); // error: "expected null, but was:<null>"
    }
}

Expected behavior
The String property should be deserialized to a literal null when null is passed in

Additional context
Similar to #3008, but using EXISTING_PROPERTY instead. Abstraction is also used in the above code example

[cowtowncoder]

One quick note: since 2.12.5 is the latest patch, could you please verify issue still exists (likely does, just want to double-check as there are many fixes in 2.12.3, 4 and 5).

[jonc2]

Verified that the issue still exists on 2.12.5.

Last test testDeserializationNull is still failing.

[cowtowncoder]

.... and this seems to have caused a regression :-( :-( :-( (see #3313)