getenv: return NULL if empty #2574

Making an environment variable empty can be a way of unsetting it for
platforms that don't support unsetenv(). In most cases, we treat empty
variables as having been unset. For all others, use os_env_exists().

src/nvim/tui/tui.c

@@ -15,6 +15,7 @@
 #include "nvim/api/private/helpers.h"
 #include "nvim/os/event.h"
 #include "nvim/tui/tui.h"
+#include "nvim/strings.h"
 
 // Space reserved in the output buffer to restore the cursor to normal when
 // flushing. No existing terminal will require 32 bytes to do that.