Fix CLI 'get' command treating empty values as missing

The 'get' command uses 'if stored_value:' to check whether a key
exists. This is a truthiness check that evaluates to False for empty
strings, so 'KEY=' or 'KEY=""' causes the command to exit with
code 1 as if the key doesn't exist.

Use 'is not None' instead to correctly distinguish between a key
set to an empty string and a key that is not present.

Author: bysiber

src/dotenv/cli.py

@@ -136,7 +136,7 @@ def get(ctx: click.Context, key: Any) -> None:
         values = dotenv_values(stream=stream)
 
     stored_value = values.get(key)
-    if stored_value:
+    if stored_value is not None:
         click.echo(stored_value)
     else:
         sys.exit(1)