TropicalGeometry: fixed initials w.r.t. t-adic valuation

[YueRen]

Fixes the following error (old code tried to convert elements of the rational function field with denominator 1 to the coefficient field, their numerators should be evaluated at 0 instead):

julia> K,t = rational_function_field(QQ,"t")
(Rational function field over QQ, t)

julia> nu = tropical_semiring_map(K,t)
Map into Min tropical semiring encoding the t-adic valuation on Rational function field over QQ

julia> initial(1+t,nu)
ERROR: InexactError: QQFieldElem(t + 1)
Stacktrace:
 [1] (::QQField)(x::AbstractAlgebra.Generic.FracFieldElem{QQPolyRingElem})
   @ Oscar ~/.julia/dev/Oscar/src/Rings/FunctionField.jl:71
 [2] QQField
   @ ~/.julia/dev/Oscar/src/Rings/FunctionField.jl:76 [inlined]
 [3] initial(c::AbstractAlgebra.Generic.RationalFunctionFieldElem{…}, nu::TropicalSemiringMap{…})
   @ Oscar ~/.julia/dev/Oscar/src/TropicalGeometry/semiring_map.jl:257
 [4] top-level scope
   @ REPL[291]:1
Some type information was truncated. Use `show(err)` to see complete types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.58%. Comparing base (61bf7dc) to head (481ace5).
Report is 21 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4040   +/-   ##
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  Coverage   84.58%   84.58%           
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  Files         597      597           
  Lines       82265    82265           
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  Hits        69587    69587           
  Misses      12678    12678           

Files with missing lines	Coverage Δ
src/TropicalGeometry/semiring_map.jl	97.22% <100.00%> (ø)

[VictoriaSchleis]

looks good