[mlir] GreedyPatternRewriteDriver: Ignore scope when rewriting top-level ops

Top-level ModuleOps cannot be transformed with the GreedyPatternRewriteDriver since D141945 because they do not have an enclosing region that could be used as a scope. Make the scope optional inside GreedyPatternRewriteDriver, so that top-level ops can be processed when they are on the initial list of ops.

Note: This does not allow users to bypass the scoping mechanism by setting `config.scope = nullptr`.

Fixes #60462.

Differential Revision: https://reviews.llvm.org/D143151