Silently ignore empty path elements

resolve_paths function used to print a warning if an input path
was empty. This commit fixes this by explicitly skipping
empty paths during path resolution.

Test: Run adb shell LD_LIBRARY_PATH=: /system/bin/ping
Test: make sure there are no warnings
Bug: http://b/31346121
Change-Id: Ifd79040943dc62fc6e07d9828ff91d1050572809