Handle trivial input better in 14.longest-common-prefix.jl

[GHTaarn]

This change makes the algorithm able to correctly handle cases where there is only one input string or where all input strings are equal.

e.g.

using Test, LeetCode

@test longest_common_prefix(["abc"]) == "abc"
@test longest_common_prefix(["abc", "abc"]) == "abc"