Bug in LCP code for empty string testcase

In the LCP code in file "cpp\strings\suffix-array.cpp" there is no handling of empty string (edge case) causing the following error 
"""
terminate called after throwing an instance of 'std::length_error'
  what():  cannot create std::vector larger than max_size()
"""
Can be fixed by a simple "if" check before vector allocation. I will create a PR with this fix.