fix ArrayIndexOutOfBoundsException in DnsStringLookup for '|' key

[dxbjavid]

DnsStringLookup.lookup splits the key on | and reads keys[0] before it checks the array length, so a key made up of only delimiter or whitespace characters (for example the interpolation ${dns:|}, or ||) splits to an empty array and throws an ArrayIndexOutOfBoundsException straight out of the public lookup instead of resolving to nothing. Every other split-based lookup in this package (FileStringLookup, UrlStringLookup, PropertiesStringLookup, XmlStringLookup, ScriptStringLookup, ResourceBundleStringLookup) guards the length before indexing, so this is the odd one out. I've returned null for that case, which is what the method already does for a null key and for a host that will not resolve, so the behaviour stays consistent and no valid key changes.

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