Fix the default for async finalizer

[pditommaso]

The PR fixes the setting for the NXF_ENABLE_ASYNC_FINALIZER env variable.

The expression 'some string' as boolean is always true unless the string is null or empty.

Therefore the setting is always enabled unless NXF_ENABLE_ASYNC_FINALIZER is set to an empty string.

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[pditommaso]

Wonder if it should be set to true by default at this point

[bentsherman]

Might be better to make it true by default and allow people to disable it if it breaks something for them. We'll get much better testing feedback that way

[pditommaso]

ok