[llvm-profdata] Fix bug llvm-profdata crashes when reading a text sample...

[llvm-profdata] Fix bug llvm-profdata crashes when reading a text sample profile with an empty line with spaces.

Text editors can introduce spaces aligning the previous line's indentation. This crashes llvm-profdata. Added check to handle this case.

Reviewed By: snehasish

Differential Revision: https://reviews.llvm.org/D143369