[CMake] fix building with zero plugins

[Cmake] fix null plugins

fix null

plugins/input/CMakeLists.txt

@@ -52,6 +52,8 @@ endif()
 # Copy all plugin dlls, so that these are in the main output dir, since cmake copies those into ${MAPNIK_OUTPUT_DIR}/plugins/input, too.
 #
 if(WIN32)
+    list(LENGTH m_build_plugins m_number_plugins)
+    if(m_number_plugins GREATER 0)
         string(CONFIGURE
             [[
                 file(GLOB m_deps "./*.dll")
@@ -68,4 +70,5 @@ if(WIN32)
             COMMENT "copying plugin dlls into build dir"
         )
         add_dependencies(copy_plugin_dependencies ${m_build_plugins})
+    endif()
 endif()