Try to avoid empty string

- g++ has a problem with ''
- clang++ does not seem to mind it.
diff --git a/meson.build b/meson.build
index 058d55a..a338cc0 100644
--- a/meson.build
+++ b/meson.build
@@ -50,8 +50,8 @@
   dll_export_flag = '-DJSON_DLL_BUILD'
   dll_import_flag = '-DJSON_DLL'
 else
-  dll_export_flag = ''
-  dll_import_flag = ''
+  dll_export_flag = []
+  dll_import_flag = []
 endif
 
 jsoncpp_lib = library(