Fix rational division by zero

[blegat]

On Julia v1.5.3:

julia> a = big(1 // 1)
1//1

julia> a / zero(a)
1//0

On Julia v1.6.0

julia> a = big(1 // 1)
1//1

julia> a / zero(a)
ERROR: MethodError: no method matching //(::BigFloat, ::BigInt)
Closest candidates are:
  //(::Integer, ::Integer) at rational.jl:61
  //(::Rational, ::Integer) at rational.jl:63
  //(::Complex, ::Real) at rational.jl:77
simonbyrne   ...
Stacktrace:
 [1] //(x::Rational{BigInt}, y::Rational{BigInt})
   @ Base.GMP.MPQ ./gmp.jl:934
 [2] /(x::Rational{BigInt}, y::Rational{BigInt})
   @ Base ./rational.jl:321
 [3] top-level scope
   @ REPL[2]:1

It seems the line was not covered by tests so the PR both fixes the error and adds some tests.

Note that the behavior is still different from Julia v1.5.3 but there is no error and it seems to be the behavior intended by @simonbyrne in #38520

[sostock]

The test failures are due to the type-instability (before this PR, the BigFloat-branch of the method always errored, so it didn’t affect the return type).

[simonbyrne]

Good catch, thanks!