FindPkgConfig: do not always rerun pkg_check_modules() with 2 arguments

Since commit v3.8.0-rc1~47^2 (FindPkgConfig: Recheck pkg-config on
parameter change, 2017-01-17) calling `pkg_check_modules` always
re-runs.  The problem is that if there are only 2 arguments passed to
`pkg_check_modules```, then `_module0` will be set and `ARGN` will be
empty.  When this is written to cache it will be stored as just the
value of `_module0` without any semicolon, so on the next run this
doesn't match the expected value and the search is rerun.

Revise the logic to handle the case of empty `ARGN` separately.

Fixes: #17003

Modules/FindPkgConfig.cmake

@@ -584,7 +584,9 @@ endmacro()
 macro(pkg_check_modules _prefix _module0)
   _pkgconfig_parse_options(_pkg_modules _pkg_is_required _pkg_is_silent _no_cmake_path _no_cmake_environment_path _imp_target "${_module0}" ${ARGN})
   # check cached value
-  if (NOT DEFINED __pkg_config_checked_${_prefix} OR __pkg_config_checked_${_prefix} LESS ${PKG_CONFIG_VERSION} OR NOT ${_prefix}_FOUND OR NOT "${__pkg_config_arguments_${_prefix}}" STREQUAL "${_module0};${ARGN}")
+  if (NOT DEFINED __pkg_config_checked_${_prefix} OR __pkg_config_checked_${_prefix} LESS ${PKG_CONFIG_VERSION} OR NOT ${_prefix}_FOUND OR
+      (NOT "${ARGN}" STREQUAL "" AND NOT "${__pkg_config_arguments_${_prefix}}" STREQUAL "${_module0};${ARGN}") OR
+      (    "${ARGN}" STREQUAL "" AND NOT "${__pkg_config_arguments_${_prefix}}" STREQUAL "${_module0}"))
     _pkg_check_modules_internal("${_pkg_is_required}" "${_pkg_is_silent}" ${_no_cmake_path} ${_no_cmake_environment_path} ${_imp_target} "${_prefix}" ${_pkg_modules})
 
     _pkgconfig_set(__pkg_config_checked_${_prefix} ${PKG_CONFIG_VERSION})