export: Regression in support for empty TARGETS list
CMake 3.15.0-rc{1,2} fails on this case:
cmake_minimum_required(VERSION 3.14)
project(Issue NONE)
export(TARGETS FILE export.cmake)
with the error
CMake Error at CMakeLists.txt:3 (export):
export EXPORT or TARGETS specifier missing.
CMake 3.14 accepted it and produced an export file with no targets.