Correctly canonicalize lifecycle block when missing hook value (#26285)

.changelog/26285.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+jobspec: Validate required hook field in lifecycle block
+```

api/tasks.go

@@ -747,13 +747,13 @@ const (
 )
 
 type TaskLifecycle struct {
-	Hook    string `mapstructure:"hook" hcl:"hook,optional"`
+	Hook    string `mapstructure:"hook" hcl:"hook"`
 	Sidecar bool   `mapstructure:"sidecar" hcl:"sidecar,optional"`
 }
 
 // Determine if lifecycle has user-input values
 func (l *TaskLifecycle) Empty() bool {
-	return l == nil || (l.Hook == "")
+	return l == nil
 }
 
 // Task is a single process in a task group.

api/tasks_test.go

@@ -344,10 +344,36 @@ func TestTask_Canonicalize_TaskLifecycle(t *testing.T) {
 		{
 			name: "empty",
 			task: &Task{
-				Lifecycle: &TaskLifecycle{},
+				Lifecycle: nil,
 			},
 			expected: nil,
 		},
+		{
+			name: "missing hook",
+			task: &Task{
+				Lifecycle: &TaskLifecycle{},
+			},
+			expected: &TaskLifecycle{},
+		},
+		{
+			name: "with sidecar",
+			task: &Task{
+				Lifecycle: &TaskLifecycle{
+					Sidecar: true,
+				},
+			},
+			expected: &TaskLifecycle{Sidecar: true},
+		},
+		{
+			name: "valid",
+			task: &Task{
+				Lifecycle: &TaskLifecycle{
+					Hook:    "prestart",
+					Sidecar: true,
+				},
+			},
+			expected: &TaskLifecycle{Hook: "prestart", Sidecar: true},
+		},
 	}
 
 	for _, tc := range testCases {