PATCH branch-protection updates check list even when checks are disabled (#26351)

Fixes: #26333.

Previously, this endpoint only updates the `StatusCheckContexts` field
when `EnableStatusCheck==true`, which makes it impossible to clear the
array otherwise.

This patch uses slice `nil`-ness to decide whether to update the list of
checks. The field is ignored when either the client explicitly passes in
a null, or just omits the field from the json ([which causes
`json.Unmarshal` to leave the struct field
unchanged](https://go.dev/play/p/Z2XHOILuB1Q)). I think this is a better
measure of intent than whether the `EnableStatusCheck` flag was set,
because it matches the semantics of other field types.

Also adds a test case. I noticed that [`testAPIEditBranchProtection`
only checks the branch
name](c1c83dbaec/tests/integration/api_branch_test.go (L68))
and no other fields, so I added some extra `GET` calls and specific
checks to make sure the fields are changing properly.

I added those checks the existing integration test; is that the right
place for it?

routers/api/v1/repo/branch.go

@@ -768,7 +768,8 @@ func EditBranchProtection(ctx *context.APIContext) {
 	if form.EnableStatusCheck != nil {
 		protectBranch.EnableStatusCheck = *form.EnableStatusCheck
 	}
-	if protectBranch.EnableStatusCheck {
+
+	if form.StatusCheckContexts != nil {
 		protectBranch.StatusCheckContexts = form.StatusCheckContexts
 	}