cmake: Fixes plugins not being copied into application bundle on macOS

Makes the check for the number of plugins in the global list an explicit
length check (not relying on CMake implicit functionality), also moves
it in line with how the same was solved in `ObsHelpers.cmake` globally.

cmake/Modules/ObsHelpers_macOS.cmake

@@ -265,7 +265,8 @@ endfunction()
 function(setup_obs_modules target)
 
   get_property(OBS_MODULE_LIST GLOBAL PROPERTY OBS_MODULE_LIST)
-  if("${OBS_MODULE_LIST}")
+  list(LENGTH OBS_MODULE_LIST _LEN)
+  if(_LEN GREATER 0)
     add_dependencies(${target} ${OBS_MODULE_LIST})
 
     install(
@@ -284,7 +285,8 @@ function(setup_obs_modules target)
 
   get_property(OBS_SCRIPTING_MODULE_LIST GLOBAL
                PROPERTY OBS_SCRIPTING_MODULE_LIST)
-  if("${OBS_SCRIPTING_MODULE_LIST}")
+  list(LENGTH OBS_SCRIPTING_MODULE_LIST _LEN)
+  if(_LEN GREATER 0)
     add_dependencies(${target} ${OBS_SCRIPTING_MODULE_LIST})
 
     install(