test: cover empty bool source validation in TestFlagsFromEmptyEnv

Only the empty env and empty file cases of
TestBoolFlagEmptySourceRunsValidator fail without the fix. The other ten
already pass on main, and a file source goes through the same PostParse
loop as an env var.

Replace the file with one case in TestFlagsFromEmptyEnv, next to the
existing "an empty bool still reads as false" case.

Assisted-by: claude:claude-opus-5-5
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

Author: Juneezee

flag_empty_bool_validator_test.go

@@ -734,6 +734,17 @@ func TestFlagsFromEmptyEnv(t *testing.T) {
 			wantValue: false,
 			wantIsSet: true,
 		},
+		{
+			name: "an empty bool still runs the validator",
+			env:  map[string]string{"DEBUG": ""},
+			fl: &BoolFlag{Name: "debug", Value: true, Sources: EnvVars("DEBUG"), Validator: func(b bool) error {
+				if !b {
+					return errors.New("debug must be true")
+				}
+				return nil
+			}},
+			errContains: "debug must be true",
+		},
 	}
 
 	for _, tc := range testCases {