fix(kscan): Allow composite driver to handle missing children.

For split keyboards using an IO expander over TRRS/i2c, if the
right half isn't connected, we should be able to gracefully
fallback to the left side still working.

app/drivers/kscan/kscan_composite.c

@@ -41,7 +41,12 @@ static int kscan_composite_enable_callback(const struct device *dev) {
     for (int i = 0; i < ARRAY_SIZE(kscan_composite_children); i++) {
         const struct kscan_composite_child_config *cfg = &kscan_composite_children[i];
 
-        kscan_enable_callback(device_get_binding(cfg->label));
+        const struct device *dev = device_get_binding(cfg->label);
+        if (!dev) {
+            LOG_WRN("Failed to load child kscan device %s", log_strdup(cfg->label));
+            continue;
+        }
+        kscan_enable_callback(dev);
     }
     return 0;
 }
@@ -50,7 +55,12 @@ static int kscan_composite_disable_callback(const struct device *dev) {
     for (int i = 0; i < ARRAY_SIZE(kscan_composite_children); i++) {
         const struct kscan_composite_child_config *cfg = &kscan_composite_children[i];
 
-        kscan_disable_callback(device_get_binding(cfg->label));
+        const struct device *dev = device_get_binding(cfg->label);
+        if (!dev) {
+            LOG_WRN("Failed to load child kscan device %s", log_strdup(cfg->label));
+            continue;
+        }
+        kscan_disable_callback(dev);
     }
     return 0;
 }