Update lib_deps.rst

[nickyonge]

specifying "can be left blank", just as a point of reference. I left it blank on a copy+paste platformio.ini template I made, and it successfully compiles. I had to test it to see - it'd be nice to know beforehand if blank is okay for compilation, adding this for the next guy :)

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[ivankravets]

What is the reason to set an option with the empty value?