OpenFAST_IO error if BDBldFile is empty

[abhineet-gupta]

Bug description
OpenFAST_IO tries to read non-existent beamdyn files

To Reproduce
Try to read a case using openfast_io/FAST_reader.py where CompElast=1 and BDBldFile(:) = ""

Expected behavior
OpenFAST_IO should skip reading any BeamDyn files.

OpenFAST Version
OpenFAST_IO 4.0.3

Suggested Fix
This issue can be fixed by changing os.path.exists to os.path.isfile in this line:

openfast/openfast_io/openfast_io/FAST_reader.py

Line 3257 in 20632d7

if os.path.exists(bd_file1):

[andrew-platt]

Fixed in 4.0.4