Use the checkFileExists() function that has been created, make sure not to try and access a character in an empty line.

Remove duplicate check that the FRDM file exists. It is done as soon as we enter the createDripLineile() function, so
no need to do it just before.
Apply the changes in commit b0831832aad that were for the single particle drip lines to the two particle drip lines function.
Lessen the scope as it's only used in the for loop.