- to support empty strings and arrays, generate property value attribute
if body content absent, and vice-versa
- for empty array members, generate empty property element (no value
attribute)

By spec, if a property element has a value attribute, then it's a
single-value property; otherwise it's a multi-value (array) property
where each non-empty line in its body, trimmed, represents an array
element.

DSEditor considers empty value and content the same as no value/content.
The annotation processor, on the other hand, actually removes the value
attribute and/or body content node to distinguish between single and
multi-value properties. Thus, when writing out the property element XML,
we must write the value attribute or body content node if it's defined,
even when empty.

Change-Id: I4a0eb93240124e1a11cb5816e96af1357df78b92
Signed-off-by: Peter Nehrer <pnehrer@eclipticalsoftware.com>