Change the contract of CoroutineContext.isActive to return 'true' for contexts with no job in it.

Otherwise, the API is becoming an error prone for being called from jobless entrypoints (i.e. 'suspend fun main' or Ktor handlers), as 'if (ctx.isActive)' is a well-established pattern for busy-wait or synchronous job.
It is now aligned with CoroutineScope.isActive behaviour.

Fixes #3300

Author: qwwdfsad

kotlinx-coroutines-core/common/src/Job.kt

@@ -539,7 +539,7 @@ public fun Job.cancelChildren(cause: Throwable? = null) {
 
 /**
  * Returns `true` when the [Job] of the coroutine in this context is still active
- * (has not completed and was not cancelled yet).
+ * (has not completed and was not cancelled yet) or the context does not have a [Job] in it.
  *
  * Check this property in long-running computation loops to support cancellation
  * when [CoroutineScope.isActive] is not available:
@@ -550,11 +550,11 @@ public fun Job.cancelChildren(cause: Throwable? = null) {
  * }
  * ```
  *
- * The `coroutineContext.isActive` expression is a shortcut for `coroutineContext[Job]?.isActive == true`.
+ * The `coroutineContext.isActive` expression is a shortcut for `get(Job)?.isActive ?: true`.
  * See [Job.isActive].
  */
 public val CoroutineContext.isActive: Boolean
-    get() = this[Job]?.isActive == true
+    get() = get(Job)?.isActive ?: true
 
 /**
  * Cancels [Job] of this context with an optional cancellation cause.

kotlinx-coroutines-core/common/test/CoroutineScopeTest.kt

@@ -277,4 +277,15 @@ class CoroutineScopeTest : TestBase() {
 
     private fun scopePlusContext(c1: CoroutineContext, c2: CoroutineContext) =
         (ContextScope(c1) + c2).coroutineContext
+
+    @Test
+    fun testIsActiveWithoutJob() {
+        var invoked = false
+        suspend fun testIsActive() {
+            assertTrue(coroutineContext.isActive)
+            invoked = true
+        }
+        ::testIsActive.startCoroutine(Continuation(EmptyCoroutineContext){})
+        assertTrue(invoked)
+    }
 }