Additional V+ pin on ALD1106
Other devices in the series have PMOS transistors and the same pinout for consistency; V+ is used for the substrate contact in those particular cases while V- might be unused. For example, the ALD1105 contains both FET types and uses both pins, while the ALD1107 contains only pFETs and leaves the V- pin unconnected like the question asks. All of these parts put these substrate pins in the same locations on the package. 

If the devices are "just" FETs, it should be safe to leave them unconnected. However, given the possiblity that they are used for some kind of ESD protection it may be wiser to connect them to your positive rail, as long as the rail voltage is within the chip's stated/implied operating range of +2 to +12 volts.