Trend visualization fix for trivial trend (#19635)

Trivial trends previously broke. Now they default to basically the ordinary single value viz.

src/metabase/pulse/render/body.clj

@@ -654,7 +654,7 @@
      :render/text (str value)}))
 
 (s/defmethod render :smartscalar :- common/RenderedPulseCard
-  [_ _ timezone-id _card _ {:keys [cols _rows insights viz-settings]}]
+  [_ _ timezone-id _card _ {:keys [cols rows insights viz-settings] :as data}]
   (letfn [(col-of-type [t c] (or (isa? (:effective_type c) t)
                                  ;; computed and agg columns don't have an effective type
                                  (isa? (:base_type c) t)))
@@ -684,7 +684,17 @@
            :render/text (str value "\n"
                              adj " " (percentage last-change) "."
                              " Was " previous " last " (format-unit unit))})
-        @error-rendered-info))))
+        ;; In other words, defaults to plain scalar if we don't have actual changes
+        {:attachments nil
+         :content     [:div
+                       [:div {:style (style/style (style/scalar-style))}
+                        (h last-value)]
+                       [:p {:style (style/style {:color         style/color-text-medium
+                                                 :font-size     :16px
+                                                 :font-weight   700
+                                                 :padding-right :16px})}
+                        (trs "Nothing to compare to.")]]
+         :render/text (str last-value "\n" (trs "Nothing to compare to."))}))))
 
 (s/defmethod render :sparkline :- common/RenderedPulseCard
   [_ render-type timezone-id card _ {:keys [_rows cols viz-settings] :as data}]

test/metabase/pulse/render/body_test.clj

@@ -314,9 +314,25 @@
                                  :unit :month
                                  :last-change 1.333333
                                  :col "value"
-                                 :last-value 40.0}]}]
+                                 :last-value 40.0}]}
+            ;; by "dumb" it is meant "without nonnil insights"
+            dumbres {:cols [{:name         "value",
+                             :display_name "VALUE",
+                             :base_type    :type/Decimal}
+                            {:name           "time",
+                             :display_name   "TIME",
+                             :base_type      :type/DateTime
+                             :effective_type :type/DateTime}]
+                     :rows [[20.0 :month-before]]
+                     :insights [{:previous-value nil
+                                 :unit nil
+                                 :last-change nil
+                                 :col "value"
+                                 :last-value 20.0}]}]
         (is (= "40.00\nUp 133.33%. Was 30.00 last month"
                (:render/text (body/render :smartscalar nil pacific-tz nil nil results))))
+        (is (= "20.0\nNothing to compare to."
+               (:render/text (body/render :smartscalar nil pacific-tz nil nil dumbres))))
         (is (schema= {:attachments (s/eq nil)
                       :content     (s/pred vector? "hiccup vector")
                       :render/text (s/eq "40.00\nUp 133.33%. Was 30.00 last month")}