Remove unnecessary early guard

Read all available bytes in batches via read_array() instead of
byte-at-a-time read() calls. Each read() internally chains through
read_byte -> read_array(1) -> check_read_timeout_ -> available(),
resulting in 3 UART calls per byte. At 256000 baud with ~235 bytes
per loop iteration, this was ~706 UART operations per loop call.
Batching reduces this to ~12.

Measured 33% reduction in loop time (2348ms -> 1577ms per 60s).

esphome/components/ld2450/ld2450.cpp

@@ -276,8 +276,19 @@ void LD2450Component::dump_config() {
 }
 
 void LD2450Component::loop() {
-  while (this->available()) {
-    this->readline_(this->read());
+  // Read all available bytes in batches to reduce UART call overhead.
+  int avail = this->available();
+  uint8_t buf[MAX_LINE_LENGTH];
+  while (avail > 0) {
+    size_t to_read = std::min(static_cast<size_t>(avail), sizeof(buf));
+    if (!this->read_array(buf, to_read)) {
+      break;
+    }
+    avail -= to_read;
+
+    for (size_t i = 0; i < to_read; i++) {
+      this->readline_(buf[i]);
+    }
   }
 }