Allow passing the field of rational numbers in maple interface

[wegank]

fc = 0 was previously not handled in MSolveGroebner, causing an undefined field_char and subsequent failures when passed to ToMSolve.

Tested with the following lines, saved as test.mpl in the project root directory, with a global installation of msolve on macOS.

read("interfaces/msolve-to-maple-file-interface-macos.mpl");
MSolveGroebner([x^2 + y^2 - 4, x * y - 1], 0, [x, y], ["mspath" = "msolve"]);