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

[GHTaarn]

Submitted here by error, the corrected version is JuliaCN#172

"master" on the fork should always follow "master" on the original