Make Subscriber interface's _next/error/complete algorithm_ non-nul…

…lable

Now, we use `Subscriber.active` to know the `Subscriber` never calls
any of the callbacks it owns after it has been closed.

So we don't have to use the way to check whether their algorithms are null or
not to know `Subscriber` is still active or not.

spec.bs

@@ -164,13 +164,13 @@ interface Subscriber {
 </xmp>
 
 Each {{Subscriber}} has a <dfn for=Subscriber>next algorithm</dfn>, which is a [=internal
-observer/next steps=]-or-null.
+observer/next steps=].
 
 Each {{Subscriber}} has a <dfn for=Subscriber>error algorithm</dfn>, which is an [=internal
-observer/error steps=]-or-null.
+observer/error steps=].
 
 Each {{Subscriber}} has a <dfn for=Subscriber>complete algorithm</dfn>, which is a [=internal
-observer/complete steps=]-or-null.
+observer/complete steps=].
 
 Each {{Subscriber}} has a <dfn for=Subscriber>teardown callbacks</dfn>, which is a [=list=] of
 {{VoidFunction}}s, initially empty.
@@ -203,8 +203,7 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
     1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
        Document=] is not [=Document/fully active=], then return.
 
-    1. If [=this=]'s [=Subscriber/next algorithm=] is not null, then run [=this=]'s
-       [=Subscriber/next algorithm=] given |value|.
+    1. Run [=this=]'s [=Subscriber/next algorithm=] given |value|.
 
        [=Assert=]: No <a spec=webidl lt="an exception was thrown">exception was thrown</a>.
 
@@ -227,19 +226,15 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
     1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
        Document=] is not [=Document/fully active=], then return.
 
-    1. Let |error algorithm| be [=this=]'s [=Subscriber/error algorithm=].
-
     1. [=close a subscription|Close=] [=this=].
 
     1. [=AbortController/Signal abort=] [=this=]'s [=Subscriber/complete or error controller=].
 
-    1. If |error algorithm| is not null, then run |error algorithm| given |error|.
+    1. Run [=this=]'s [=Subscriber/error algorithm=] given |error|.
 
        [=Assert=]: No <a spec=webidl lt="an exception was thrown">exception was thrown</a>.
 
        Note: See the documentation in {{Subscriber/next()}} for details on why this is true.
-
-    1. Otherwise (i.e., when |error algorithm| is null), [=report the exception=] |error|.
 </div>
 
 <div algorithm>
@@ -250,13 +245,11 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
     1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
        Document=] is not [=Document/fully active=], then return.
 
-    1. Let |complete algorithm| be [=this=]'s [=Subscriber/complete algorithm=].
-
     1. [=close a subscription|Close=] [=this=].
 
     1. [=AbortController/Signal abort=] [=this=]'s [=Subscriber/complete or error controller=].
 
-    1. If |complete algorithm| is not null, then run |complete algorithm|.
+    1. Run [=this=]'s [=Subscriber/complete algorithm=].
 
        [=Assert=]: No <a spec=webidl lt="an exception was thrown">exception was thrown</a>.
 
@@ -283,9 +276,6 @@ The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to
 
     1. Set |subscriber|'s [=Subscriber/active=] boolean to false.
 
-    1. Set |subscriber|'s [=Subscriber/next algorithm=], [=Subscriber/error algorithm=], and
-       [=Subscriber/complete algorithm=] all to null.
-
   <div class=note>
     <p>This algorithm intentionally does not have script-running side-effects; it just updates the
     internal state of a {{Subscriber}}. It's important that this algorithm sets