Add ObjectUtils.nullSafeGetValue methods for null-safe value retrieval

[lucky8987]

ObjectUtils already provides a family of null-safe utilities such as
nullSafeEquals, nullSafeHashCode, and nullSafeToString. However, when
extracting a value from a potentially null object — for example, reading a
property from a header, attribute, or configuration value — there is currently
no equivalent convenience method, and callers have to write repetitive manual
null checks:

Object value = (obj != null ? obj.getName() : null);

or

  Object value = (obj != null && obj.getName() != null ? obj.getName() : "default");

Approach

This commit introduces two nullSafeGetValue methods to ObjectUtils:

• nullSafeGetValue(T, Function<T, R>) — retrieves a value by applying the
  supplied get function to the given object, returning null if the given
  object is null.
• nullSafeGetValue(T, Function<T, R>, R) — additionally accepts a default
  value which is returned if the given object is null or the supplied get
  function produced a null value.

Since the get function receives the object as its argument and is only invoked
when the object is non-null, the methods structurally guarantee that the
function is never applied to a null object — there is no risk of the
function dereferencing a captured null reference. This complements the
existing nullSafeEquals, nullSafeHashCode, and nullSafeToString family
of methods.

Looking forward to seeing this PR merged soon!

[bclozel]

Thanks for the proposal but such utility classes are not meant to be broadly used by applications: their first goal must be to be used within the Framework. We didn't really miss this so far so I'm going to decline it.