[Dotenv] FIX missing getenv

Author: mccullagh

Dotenv.php

@@ -387,7 +387,7 @@ private function resolveVariables($value, array $loadedVars)
             } elseif (isset($this->values[$name])) {
                 $value = $this->values[$name];
             } else {
-                $value = '';
+                $value = (string) getenv($name);
             }
 
             if (!$matches['opening_brace'] && isset($matches['closing_brace'])) {

Tests/DotenvTest.php

@@ -314,4 +314,15 @@ public function testGetVariablesValueFromEnvFirst()
             $this->assertSame('foo2_prod', $values['TEST2']);
         }
     }
+
+    public function testGetVariablesValueFromGetenv()
+    {
+        putenv('Foo=Bar');
+
+        $dotenv = new Dotenv(true);
+        $values = $dotenv->parse('Foo=${Foo}');
+        $this->assertSame('Bar', $values['Foo']);
+
+        putenv('Foo');
+    }
 }