default value for configurable bootclasspath in javac

Hi. I've got the following in my build xml:

	<property name="javac.bootclasspath" value="" />

	<presetdef name="myjavac">
		<javac source="1.7" target="1.7" includeantruntime="no" bootclasspath="${javac.bootclasspath}" >
		</javac>
	</presetdef>

The idea is to not alter bootclasspath when the caller did not provide an explicit -Djavac.bootclasspath=...
An empty string as the default value works fine with Sun Javac, but it fails with: <javac compiler="org.eclipse.jdt.core.JDTCompilerAdapter">

     [myjavac] The type java.lang.Double cannot be resolved. It is indirectly referenced from required .class files
     [myjavac] ----------
     [myjavac] 1 problem (1 error)

Since the presetdef task does not support conditional attributes I'm asking for an advice on how to make it work.

This is Ant 1.10.8 bundled with Eclipse.