allow construction of empty signed permutation

[mantepse]

Define signed permutations on zero elements:

sage: S = SignedPermutations(0)
sage: S
Signed permutations of 0

Before this ticket:

sage: S([])
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
...
~/sage-develop/local/lib/python3.8/site-packages/sage/combinat/colored_permutations.py in _element_constructor_(self, x)
   1190         """
   1191         if isinstance(x, list):
-> 1192             if isinstance(x[0], tuple):
   1193                 c = []
   1194                 p = []

IndexError: list index out of range

After this ticket:

sage: S([])
[]

CC: @slel @tscrim

Component: combinatorics

Author: Martin Rubey

Branch/Commit: e807d8e

Reviewer: Samuel Lelièvre

Issue created by migration from https://trac.sagemath.org/ticket/30667

[mantepse]

Branch: u/mantepse/allow_construction_of_empty_signed_permutation

[mantepse]

Author: Martin Rubey

[mantepse]

Commit: e807d8e

[slel]

Reviewer: Samuel Lelièvre

[slel]

comment:3

Good.

[tscrim]

comment:6

Yay corner cases! Good catch and fix. Thanks.

[vbraun]

Changed branch from u/mantepse/allow_construction_of_empty_signed_permutation to e807d8e