Deprecation of empty resolver arguments in favor of explicit strings

[odelalleau]

In OmegaConf 2.0, it is possible to have empty elements in a list of resolver arguments, for instance:

• ${foo:a,} calls foo("a", "")
• ${foo:a,,b} calls foo("a", "", "b")
• ${foo:,} calls foo("", "")

This has been deprecated in OmegaConf 2.1. Users should instead use explicit empty strings, for instance:

• ${foo:a,""}
• ${foo:a, '', b}
• ${foo:"", ""}