allow unset PATH

[simonLeary42]

$ bash -c '. /usr/share/lmod/lmod/init/profile; unset PATH; module --version'
/usr/bin/lua5.3: /usr/share/lmod/lmod/libexec/../tools/fileOps.lua:76: attempt to index a nil value (local 'path')
stack traceback:
	/usr/share/lmod/lmod/libexec/../tools/fileOps.lua:76: in function 'findInPath'
	/usr/share/lmod/lmod/libexec/pager.lua:87: in function 'buildPager'
	/usr/share/lmod/lmod/libexec/pager.lua:97: in main chunk
	[C]: in function 'require'
	/usr/share/lmod/lmod/libexec/lmod:111: in main chunk
	[C]: in ?

[rtmclay]

Thanks very much for the patch!