[clang-rename] Exit gracefully when no input provided

clang-rename on a non existing file segfaults

Command to run -
$ clang-rename -offset=0 -new-name=plop asdasd

Error while processing llvm-project/asdasd.
clang-rename: llvm-project/llvm/include/llvm/Support/ErrorOr.h:237:
llvm::ErrorOr<T>::storage_type* llvm::ErrorOr<T>::getStorage()
[with T = const clang::FileEntry*; llvm::ErrorOr<T>::storage_type = const clang::FileEntry*]:
Assertion `!HasError && "Cannot get value when an error exists!"' failed.

[1]    827497 IOT instruction  clang-rename -offset=0 -new-name=plop asdasd

Differential Revision: https://reviews.llvm.org/D148439