"NULL" entry no longer supported for libraries in startup script

[skliper]

Describe the bug
Set sample_lib entry point to NULL:

OS_GenericSymbolLookup_Impl():118:Error: NULL: ./cf/sample_lib.so: undefined symbol: NULL
1980-012-14:04:03.48915 ES Startup: Could not find symbol:NULL. EC = 0xFFFFFFFF

To Reproduce
See above.

Expected behavior
NULL should skip the symbol lookup and load. Note it does skip if the field is just left empty, as in:
CFE_LIB, /cf/fs_lib.so, , FS_LIB, 0, 0, 0x0, 0;

Code snips
Only skips if empty string:

cFE/modules/es/fsw/src/cfe_es_apps.c

Line 453 in 82c1bd4

if (ReturnCode == CFE_SUCCESS && LoadParams->InitSymbolName[0] != 0)

Probably could just convert "NULL" to empty in the CFS_LIB processing section.

System observed on:

• Hardware: Docker
• OS: Ubuntu 18.04
• Versions: Bundle main

Additional context
Found by @excaliburtb

Reporter Info
Jacob Hageman - NASA/GSFC