STYLE: Replace comparisons to "" by empty() calls in ParameterFileParser
STYLE: Replace comparisons to "" by empty() calls in ParameterFileParser

Fixes two Clang Tidy (LLVM 9.0.0) warnings:

> warning: the 'empty' method should be used to check for emptiness instead of comparing to an empty object [readability-container-size-empty]

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix