Kernel of a homomorphism of modules over different rings

Nobody has claimed this yet.

There is a problem with computing the kernel of a homomorphism of modules defined over different rings, when the target ring has no variables.

When the target ring was QQ[] I got:

```m2
i1 : A = QQ[x]

o1 = A

o1 : PolynomialRing

i2 : B = QQ[]

o2 = B

o2 : PolynomialRing

i3 : f = map(B,A,{0})

o3 = map(B,A,{0})

o3 : RingMap B <--- A

i4 : g = map(B^1,A^1,f,{{1}})

o4 = | 1 |

i5 : ker g
stdio:5:1:(3): error: assertion failed

When the target ring was QQ I got:
i1 : A = QQ[x]

o1 = A

o1 : PolynomialRing

i2 : B = QQ

o2 = QQ

o2 : Ring

i3 : f = map(B,A,{0})

o3 = map(QQ,A,{0})

o3 : RingMap QQ <--- A

i4 : g = map(B^1,A^1,f,{{1}})

o4 = | 1 |

i5 : ker g
stdio:5:1:(3): error: no coefficient ring present
```

No source file or test is named. Start by reproducing both reported examples involving ker g, then trace the kernel implementation for maps between modules over different rings. Done means both examples compute a kernel without an assertion failure or a missing-coefficient-ring error.

Written by the indexing model from the issue text.