[online_image] Bugfix: Use std::string instead of const char * (#7556)

tests/components/online_image/common-esp32.yaml

@@ -4,13 +4,13 @@ spi:
   - id: spi_main_lcd
     clk_pin: 16
     mosi_pin: 17
-    miso_pin: 15
+    miso_pin: 18
 
 display:
   - platform: ili9xxx
     id: main_lcd
     model: ili9342
-    cs_pin: 12
+    cs_pin: 20
     dc_pin: 13
     reset_pin: 21
     invert_colors: true

tests/components/online_image/common.yaml

@@ -34,4 +34,12 @@ time:
           - online_image.set_url:
               id: online_rgba_image
               url: http://www.example.org/example.png
+          - online_image.set_url:
+              id: online_rgba_image
+              url: !lambda |-
+                return "http://www.example.org/example.png";
+          - online_image.set_url:
+              id: online_rgba_image
+              url: !lambda |-
+                return str_sprintf("http://homeassistant.local:8123");