[mlir][tablegen] Generate default attr values in Python bindings

When specifying an op attribute with a default value (via DefaultValuedAttr), the default value is a string of C++ code. In the general case, the default value of such an attribute cannot be translated to Python when generating the bindings. However, we can hard-code default Python values for frequently-used C++ default values.

This change adds a Python default value for empty ArrayAttrs.

Differential Revision: https://reviews.llvm.org/D127750