Simplify HighlightingAssets::get_syntax() first_line logic (#1852)

And make self.get_first_line_syntax() be called lazily.

tests/examples/regression_tests/first_line_fallback.invalid-syntax

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+echo "Hello"

tests/integration_tests.rs

@@ -1127,6 +1127,21 @@ fn do_not_detect_different_syntax_for_stdin_and_files() {
     );
 }
 
+#[test]
+fn no_first_line_fallback_when_mapping_to_invalid_syntax() {
+    let file = "regression_tests/first_line_fallback.invalid-syntax";
+
+    bat()
+        .arg("--color=always")
+        .arg("--map-syntax=*.invalid-syntax:InvalidSyntax")
+        .arg(&format!("--file-name={}", file))
+        .arg("--style=plain")
+        .arg(file)
+        .assert()
+        .failure()
+        .stderr(predicate::str::contains("unknown syntax: 'InvalidSyntax'"));
+}
+
 #[test]
 fn show_all_mode() {
     bat()