Fix handling of empty XDG environment variables

Closes #21132

According to the XDG Base Directory specification
(https://specifications.freedesktop.org/basedir-spec/latest/#variables),
empty values for these environment variables should be treated the same
as if they are unset. Specifically, for the instances changed in this
commit,

> $XDG_DATA_HOME defines the base directory relative to which
> user-specific data files should be stored. If $XDG_DATA_HOME is either
> not set **or empty**, a default equal to $HOME/.local/share should be
> used.

and

> $XDG_CACHE_HOME defines the base directory relative to which
> user-specific non-essential data files should be stored. If
> $XDG_CACHE_HOME is either not set **or empty**, a default equal to
> $HOME/.cache should be used.

(emphasis mine)

In addition to the case mentioned in the linked issue, all other uses of
XDG environment variables were corrected.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/introspect.zig b/src/introspect.zig
index 341b1cddeb..4193440461 100644
--- a/src/introspect.zig
+++ b/src/introspect.zig
@@ -90,8 +90,11 @@ pub fn resolveGlobalCacheDir(allocator: mem.Allocator) ![]u8 {
 
     if (builtin.os.tag != .windows) {
         if (std.zig.EnvVar.XDG_CACHE_HOME.getPosix()) |cache_root| {
-            return fs.path.join(allocator, &[_][]const u8{ cache_root, appname });
-        } else if (std.zig.EnvVar.HOME.getPosix()) |home| {
+            if (cache_root.len > 0) {
+                return fs.path.join(allocator, &[_][]const u8{ cache_root, appname });
+            }
+        }
+        if (std.zig.EnvVar.HOME.getPosix()) |home| {
             return fs.path.join(allocator, &[_][]const u8{ home, ".cache", appname });
         }
     }