[DependencyInjection] Fix support for empty env vars

Author: nicolas-grekas

EnvVarProcessor.php

@@ -149,7 +149,7 @@ public function getEnv(string $prefix, string $name, \Closure $getEnv): mixed
             || false === ($env = $env ?? getenv($name) ?? false) // null is a possible value because of thread safety issues
         ) {
             foreach ($this->loadedVars as $vars) {
-                if (false !== ($env = ($vars[$name] ?? false)) && '' !== $env) {
+                if (false !== ($env = ($vars[$name] ?? $env)) && '' !== $env) {
                     break;
                 }
             }
@@ -167,7 +167,7 @@ public function getEnv(string $prefix, string $name, \Closure $getEnv): mixed
                             continue;
                         }
                         $this->loadedVars[] = $vars = $loader->loadEnvVars();
-                        if (false !== ($env = ($vars[$name] ?? false)) && '' !== $env) {
+                        if (false !== ($env = ($vars[$name] ?? $env)) && '' !== $env) {
                             $ended = false;
                             break;
                         }

Tests/EnvVarProcessorTest.php

@@ -727,6 +727,7 @@ public static function validCsv()
     public function testEnvLoader()
     {
         $_ENV['BAZ_ENV_LOADER'] = '';
+        $_ENV['BUZ_ENV_LOADER'] = '';
 
         $loaders = function () {
             yield new class() implements EnvVarLoaderInterface {
@@ -751,7 +752,7 @@ public function loadEnvVars(): array
             };
         };
 
-        $processor = new EnvVarProcessor(new Container(), $loaders());
+        $processor = new EnvVarProcessor(new Container(), new RewindableGenerator($loaders, 2));
 
         $result = $processor->getEnv('string', 'FOO_ENV_LOADER', function () {});
         $this->assertSame('123', $result);
@@ -762,10 +763,14 @@ public function loadEnvVars(): array
         $result = $processor->getEnv('string', 'BAZ_ENV_LOADER', function () {});
         $this->assertSame('567', $result);
 
+        $result = $processor->getEnv('string', 'BUZ_ENV_LOADER', function () {});
+        $this->assertSame('', $result);
+
         $result = $processor->getEnv('string', 'FOO_ENV_LOADER', function () {});
         $this->assertSame('123', $result); // check twice
 
         unset($_ENV['BAZ_ENV_LOADER']);
+        unset($_ENV['BUZ_ENV_LOADER']);
     }
 
     public function testCircularEnvLoader()