Fixed a NPE in IPDBField.getValueString()

Since changing the internal representation of IP addresses in side the
Ganymede database from Byte[] to arlut.csd.ganymede.common.IPAddress,
the getValueString() method on IPDBField has been throwing a
NullPointerException when no value is set.

Previously, IPDBField would return "null" for an undefined scalar IPDBField
and "" for an undefined vector IPDBField.

With this change, both undefined scalar and vector IPDBFields will return
an empty string when no value has been set.
All DBField subclasses now return empty string for undefined getValueString()

Previously, the DBField subclasses were all returning "null" for
a scalar field that contained no data, and "" for a vector field
that contained no data.

Now, all DBField subclasses return an empty string when the contained
value object is null.