Fix nil pointer dereference on whole-document operations without a value

[AmazinMax]

Applying an add, replace or test operation with an empty path and no value member dereferences a nil *lazyNode and panics. This affects both v4 and v5:

Patch	v4.13.0	v5.9.11
[{"op":"replace","path":""}]	panic	rejected by DecodePatch, panic if the Patch is built directly
[{"op":"add","path":""}]	error	rejected by DecodePatch, panic if the Patch is built directly
[{"op":"test","path":""}]	panic	panic

Reproducer:

p, _ := jsonpatch.DecodePatch([]byte(`[{"op":"test","path":""}]`))
p.Apply([]byte(`{"foo":"bar"}`)) // panic: runtime error: invalid memory address or nil pointer dereference

The replace case was reported in #168, which was closed, but it still reproduces on v4.13.0. #114 / #158 fixed the same panic for test with a non-empty path, but not for the whole document.

This matters for servers that apply patches received from clients. For example, kube-apiserver applies user-supplied JSON Patch documents with v4, so any user allowed to patch a resource can trigger the panic with a single request.

Changes

• add (v5) and replace (v4, v5) with an empty path and no value return an error wrapping ErrMissing.
• test with an empty path and no value fails with ErrTestFailed. This keeps the existing semantics of test, where a missing value is treated as null (see the test cases without a value in TestCases), and the whole document is never null.
• Operations with a non-empty path are not changed.

The v4 and v5 changes are in separate commits, so either can be taken on its own.

Tests

• New entries in BadCases and TestCases for v4 and v5.
• TestMissingValueOnWholeDocument in v5 builds the Patch without DecodePatch, because DecodePatch already rejects add and replace without a value.
• Without the fix, the new cases panic in both go test runs. v4 has no go.mod and is not run in CI, so I ran its tests with a temporary go.mod (module gopkg.in/evanphx/json-patch.v4).