Fix / mitigate passing non-array subtree to printNode_C for '!' key.

[wttw]

Generating C code leads to

PHP Fatal error:  Uncaught TypeError: array_keys(): Argument #1 ($array) must be of type array, string given in /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php:118
Stack trace:
#0 /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php(118): array_keys('')
#1 /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php(133): printNode_C('!', '')
#2 /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php(133): printNode_C('!', Array)
#3 /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php(133): printNode_C('www', Array)
#4 /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php(199): printNode_C('ck', Array)
#5 {main}
  thrown in /Users/steve/3rd/registered-domain-libs/generateEffectiveTLDs.php on line 118

This PR explicitly checks for the case where the subtree is not an array and doesn't recurse. That seems to mitigate the issue, and the generated code seems to work.