`FOO-NOTFOUND` in `blt_add_executable` `DEPENDS_ON` list causes all dependencies to be lost

Having a variable in the DEPENDS_ON list of blt_add_executable causes all dependencies to not be set in the LINK_LIBRARIES target property.

Reproducer:

cmake_minimum_required(VERSION 3.3)

project(test)

set(BLT_SOURCE_DIR "/usr/workspace/white238/blt/repo")
include(${BLT_SOURCE_DIR}/SetupBLT.cmake)

blt_import_library(NAME foo
                   INCLUDES /usr/lib64)

blt_add_executable(NAME mainDoesntWork
                   SOURCES main.cpp
                   DEPENDS_ON foo BAR-NOTFOUND)

blt_add_executable(NAME mainWorks
                   SOURCES main.cpp
                   DEPENDS_ON foo)

# `mainDoesntWork` will have `LINK_LIBRARIES` defined
blt_print_target_properties(TARGET mainDoesntWork)
# `mainWorks` will have `LINK_LIBRARIES` set to `foo`
blt_print_target_properties(TARGET mainWorks)

Start at the blt_add_executable entry point and inspect how its DEPENDS_ON arguments are assigned to the target's LINK_LIBRARIES property. Run the provided reproducer and confirm that foo remains in LINK_LIBRARIES when BAR-NOTFOUND is also present; done means the valid dependency is retained.