fix: fixed footer newline behavior

[polistern]

Footer newline behavior

Without or empty footer now:

app v1
use: [-h|--help] | --option UINT

Options:
  -h,--help                             Show this message
<newline>

With footer now:

app v1
use: [-h|--help] | --option UINT

Options:
  -h,--help                             Show this message
<newline>
footer

Without or empty footer with follow changes:

app v1
use: [-h|--help] | --option UINT

Options:
  -h,--help                             Show this message

With footer (like now):

app v1
use: [-h|--help] | --option UINT

Options:
  -h,--help                             Show this message
<newline>
footer

If footer is set, then we can expect it to be at least one string.
So somebody can add more newlines directly in footer string if needed.

[codecov]

Codecov Report

Base: 99.42% // Head: 99.42% // No change to project coverage 👍

Coverage data is based on head (6bc52ed) compared to base (25274e2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #796   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          16       16           
  Lines        3977     3977           
=======================================
  Hits         3954     3954           
  Misses         23       23           

Impacted Files	Coverage Δ
include/CLI/impl/Formatter_inl.hpp	99.40% <100.00%> (ø)

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