[clang] fix undefined behaviour in RawComment::getFormattedText()

Summary:
Calling `back()` and `pop_back()` on the empty string is undefined
behavior [1,2].

The issue manifested itself as an uncaught `std::out_of_range` exception
when running `clangd` compiled on RHEL7 using devtoolset-9.

[1] https://en.cppreference.com/w/cpp/string/basic_string/back
[2] https://en.cppreference.com/w/cpp/string/basic_string/pop_back

Fixes: 1ff7c32f

Reviewers: teemperor, ioeric, cfe-commits

Reviewed By: teemperor

Subscribers: ilya-biryukov, kadircet, usaxena95

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77468

(cherry picked from commit ad7211df)