Zero value MapValue causes panic in Equal method

[jar-b]

Module version

v1.6.1

Expected Behavior

Equal handles a nil elementType. I'm not sure if the receiver having a nil type should always return false, or if an input that also has a nil type and equivalent state should technically match.

Actual Behavior

Panic.

Steps to Reproduce

1. Add a test case like the following to TestMapValueEqual.

    "zero-null": {
        receiver: MapValue{},
        input:    NewMapNull(StringType{}),
        expected: false,
    },

2. Observe panic.

% go test -v ./types/basetypes/... -run=TestMapValueEqual/zero-null
=== RUN   TestMapValueEqual
=== PAUSE TestMapValueEqual
=== CONT  TestMapValueEqual
=== RUN   TestMapValueEqual/zero-null
=== PAUSE TestMapValueEqual/zero-null
=== CONT  TestMapValueEqual/zero-null
--- FAIL: TestMapValueEqual (0.00s)
    --- FAIL: TestMapValueEqual/zero-null (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x20 pc=0x104e0eba0]

goroutine 19 [running]:
testing.tRunner.func1.2({0x104f481a0, 0x1051653c0})
        /opt/homebrew/Cellar/go/1.21.7/libexec/src/testing/testing.go:1545 +0x1c4
testing.tRunner.func1()
        /opt/homebrew/Cellar/go/1.21.7/libexec/src/testing/testing.go:1548 +0x360
panic({0x104f481a0?, 0x1051653c0?})
        /opt/homebrew/Cellar/go/1.21.7/libexec/src/runtime/panic.go:914 +0x218
github.com/hashicorp/terraform-plugin-framework/types/basetypes.MapValue.Equal({0x0?, {0x0?, 0x0?}, 0x7d?}, {0x104f9cb20?, 0x14000142240?})
        /Users/jaredbaker/development/terraform-plugin-framework/types/basetypes/map_value.go:259 +0x50
github.com/hashicorp/terraform-plugin-framework/types/basetypes.TestMapValueEqual.func1(0x14000124b60)
        /Users/jaredbaker/development/terraform-plugin-framework/types/basetypes/map_value_test.go:601 +0x58
testing.tRunner(0x14000124b60, 0x14000126bc0)
        /opt/homebrew/Cellar/go/1.21.7/libexec/src/testing/testing.go:1595 +0xe8
created by testing.(*T).Run in goroutine 18
        /opt/homebrew/Cellar/go/1.21.7/libexec/src/testing/testing.go:1648 +0x33c
FAIL    github.com/hashicorp/terraform-plugin-framework/types/basetypes 0.447s
FAIL

References

Relates hashicorp/terraform-provider-aws#36437

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