don't fail when referenced env var does not exist

Author: xabbuh

Dotenv.php

@@ -466,7 +466,7 @@ private function resolveVariables(string $value, array $loadedVars): string
                 $value = '';
             }
 
-            if ('' === $value && isset($matches['default_value'])) {
+            if ('' === $value && isset($matches['default_value']) && '' !== $matches['default_value']) {
                 $unsupportedChars = strpbrk($matches['default_value'], '\'"{$');
                 if (false !== $unsupportedChars) {
                     throw $this->createFormatException(sprintf('Unsupported character "%s" found in the default value of variable "$%s".', $unsupportedChars[0], $name));

Tests/DotenvTest.php

@@ -172,6 +172,7 @@ public function getEnvData()
             ["FOO=BAR\nBAR=\${NOTDEFINED:=TEST}", ['FOO' => 'BAR', 'NOTDEFINED' => 'TEST', 'BAR' => 'TEST']],
             ["FOO=\nBAR=\${FOO:=TEST}", ['FOO' => 'TEST', 'BAR' => 'TEST']],
             ["FOO=\nBAR=\$FOO:=TEST}", ['FOO' => 'TEST', 'BAR' => 'TEST}']],
+            ["FOO=foo\nFOOBAR=\${FOO}\${BAR}", ['FOO' => 'foo', 'FOOBAR' => 'foo']],
         ];
 
         if ('\\' !== \DIRECTORY_SEPARATOR) {