Improved types around getenv() and implicit false casts

Fixes:

```
ERROR: RiskyTruthyFalsyComparison
at src/DocbookPage.php:76:13
Operand of type false|non-empty-string contains type non-empty-string, which can be falsy and truthy. This can cause possibly unexpected behavior. Use strict comparison instead. (see https://psalm.dev/356)
        if (! $firstLine || ! str_starts_with($firstLine, '<h1>') || ! str_ends_with($firstLine, '</h1>')) {

ERROR: RiskyTruthyFalsyComparison
at src/Environment.php:30:16
Operand of type false|string contains type string, which can be falsy and truthy. This can cause possibly unexpected behavior. Use strict comparison instead. (see https://psalm.dev/356)
        return getenv($name) ?: null;
```

src/DocbookPage.php

@@ -73,7 +73,7 @@ private function determineTitleFromContent(): string
     {
         $firstLine = strtok($this->content, "\n");
 
-        if (! $firstLine || ! str_starts_with($firstLine, '<h1>') || ! str_ends_with($firstLine, '</h1>')) {
+        if ($firstLine === false || ! str_starts_with($firstLine, '<h1>') || ! str_ends_with($firstLine, '</h1>')) {
             throw new RuntimeException('First line of markdown file ' . $this->slug() . ' did not start with "# "...');
         }
 

src/Environment.php

@@ -25,9 +25,14 @@ public static function require(string $name): string
         return $value;
     }
 
+    /** @return non-empty-string|null */
     public static function optional(string $name): string|null
     {
-        return getenv($name) ?: null;
+        $value = getenv($name);
+
+        return $value === false || $value === ''
+            ? null
+            : $value;
     }
 
     /** @param list<string> $trueValues */