include(GoogleTest)

file(GLOB_RECURSE myprojectest_SRC
  "${PROJECT_SOURCE_DIR}/src/*_test.cpp"
)
add_executable(myprojecttest ${myprojecttest_SRC})
gtest_add_tests(TARGET myprojectest)

Add some files with tests in src directory: works fine.

Add an empty file. Now cmake failes with obscure errors:

CMake Error at /usr/share/cmake-3.12/Modules/GoogleTest.cmake:311 (string):
  string sub-command REGEX, mode MATCHALL needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  CMakeLists.txt:125 (gtest_add_tests)

If a source file is empty or doesn't have any test, it shouldn't fail.

Sometimes I (and I guess other developers) add some "placeholder" empty file where future tests have to be created.