sagemathgh-40388: Do not ignore env.py fallbacks for empty variables in config.py
    
If a variable is set to the empty string in config.py, consider the
fallbacks defined in env.py

In particular, this fixes MATHJAX_DIR outside sage-the-distro
    
URL: sagemath#40388
Reported by: Antonio Rojas
Reviewer(s):

Author: Release Manager

src/sage/env.py

@@ -159,9 +159,9 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
             except ImportError:
                 pass
 
-    # Try all fallbacks in order as long as we don't have a value
+    # Try all fallbacks in order as long as we don't have a non-empty value
     for f in fallbacks:
-        if value is not None:
+        if value not in (None, ""):
             break
         value = f
     SAGE_ENV[key] = value