gptel-request: Fix blank response check inside reasoning block

* gptel-request.el (gptel-curl--stream-filter): Update reasoning
block parsing logic when streaming.  Now:
1. blank response chunks before reasoning blocks are ignored by
the reasoning parser.  (Same as before)
2. blank response chunks inside a reasoning block are captured as
reasoning text by the parser. (New behavior)

Without this, reasoning text consisting of blank strings is
categorized as a response, causing the reasoning block demarcation
to fail and additional "#+begin_reasoning" (or equivalent markup)
to be inserted by gptel's default streaming callback.

Author: karthink

gptel-request.el

@@ -2764,7 +2764,8 @@ PROCESS and _STATUS are process parameters."
                   (unless reasoning-block ;Record that we're in a reasoning block (#709)
                     (plist-put proc-info :reasoning-block 'in))
                   (plist-put proc-info :reasoning nil)) ;Reset for next parsing round
-                 ((string-blank-p response)) ;Defer checking if response is blank
+                 ((and (string-blank-p response) ;Defer checking if response is blank
+                       (not reasoning-block))) ;unless we're in a reasoning block already
                  ((and (null reasoning-block) (length> response 0))
                   ;; Obtained from main response stream: reasoning block start
                   (if-let*  ((idx (string-match-p "<think>" response)))