[clang-format-diff] Fix missing formatting for zero length git diff lines

If we only delete lines that are outer block statements (if, while, etc),
clang-format-diff.py can't format the statements inside the block statements.

An example to repro:
1. Delete the if statment at line 118 in llvm/lib/CodeGen/Analysis.cpp.
2. Run `git diff -U0 --no-color HEAD^ | clang/tools/clang-format/clang-format-diff.py -i -p1`

It fails to format the statement after if.

Differential Revision: https://reviews.llvm.org/D111273