[mlir][Linalg] Allow transformation filter to match by default.

The current setup of LinalgTransformationFilter allows a
transformation to trigger when either
1) The StringAttr is not set and no filter identifier is specified.
2) The StringAttr is set and its value matches (one of) the provided
identifier.
This misses the case where the transformation should trigger either
when the attribute is not set or its value matches (one of) the
provided identifier. Since `Identifier` does not allow empty strings,
add a boolean option to match when the attribute is not set. This
option is by default off.

Differential Revision: https://reviews.llvm.org/D113057