Non-optimal behavior for collection-based configuration keys

[dmlloyd]

Both the standard List injection and our own io.smallrye.config.SmallRyeConfig#getValues use the same mechanism to expand a list of items. However neither one has any special handling for empty values so chances are things like this will fail:

// config contains: test.key=1,2,,4,5
config.getValues("test.key", Integer.class, ArrayList::new);

The problem is that if we start supporting property expansion, this kind of thing becomes possible when you have an input string like: 1,2,${maybe.three?},4,5.

I propose that we treat empty list items as not present, so the above example would result in a four-element list of [1,2,4,5]. But this would require a spec change as well because of the list support in CDI. WDYT?

[dmlloyd]

/cc @jmesnil

[dmlloyd]

A strongly related issue is the translation of empty strings into a single-element list of an empty string.