do not pass NULL to os_getenv

closes #8393

src/nvim/eval.c

@@ -15712,6 +15712,9 @@ static void get_xdg_var_list(const XDGVarType xdg, typval_T *rettv)
   rettv->vval.v_list = list;
   tv_list_ref(list);
   char *const dirs = stdpaths_get_xdg_var(xdg);
+  if (dirs == NULL) {
+    return;
+  }
   do {
     size_t dir_len;
     const char *dir;

src/nvim/main.c

@@ -1849,6 +1849,7 @@ static void source_startup_scripts(const mparm_T *const parmp)
 /// @return FAIL if the environment variable was not executed,
 ///         OK otherwise.
 static int process_env(char *env, bool is_viminit)
+  FUNC_ATTR_NONNULL_ALL
 {
   const char *initstr = os_getenv(env);
   if (initstr != NULL) {

src/nvim/option.c

@@ -6585,7 +6585,7 @@ void vimrc_found(char_u *fname, char_u *envname)
 {
   char_u      *p;
 
-  if (fname != NULL) {
+  if (fname != NULL && envname != NULL) {
     p = (char_u *)vim_getenv((char *)envname);
     if (p == NULL) {
       /* Set $MYVIMRC to the first vimrc file found. */

src/nvim/os/stdpaths.c

@@ -65,7 +65,7 @@ char *stdpaths_get_xdg_var(const XDGVarType idx)
   const char *env_val = os_getenv(env);
 
 #ifdef WIN32
-  if (env_val == NULL) {
+  if (env_val == NULL && xdg_defaults_env_vars[idx] != NULL) {
     env_val = os_getenv(xdg_defaults_env_vars[idx]);
   }
 #endif
@@ -74,7 +74,7 @@ char *stdpaths_get_xdg_var(const XDGVarType idx)
   if (env_val != NULL) {
     ret = xstrdup(env_val);
   } else if (fallback) {
-    ret = (char *) expand_env_save((char_u *)fallback);
+    ret = (char *)expand_env_save((char_u *)fallback);
   }
 
   return ret;