REL-1249-B: NullPointerException bug fix

A bug was introduced in the REL-1249 feature that adds a new GMOS Phase 2 checking rule (`IFU_NO_SPATIAL_BINNING_RULE`).  In particular, a reasonable but false assumption was made that the FPU property would always be defined in the sequence.  The reason this assumption doesn't hold has to do with the way sequence items are added.  See `InstGmosCommon.getSysConfig()` around line 1766 (!):

        GmosCommonType.FPUnitMode fpuMode = getFPUnitMode();
        sc.putParameter(DefaultParameter.getInstance(FPU_MODE_PROP.getName(), fpuMode));
        if (fpuMode == GmosCommonType.FPUnitMode.BUILTIN) {
            sc.putParameter(DefaultParameter.getInstance(FPU_PROP_NAME, getFPUnit()));
        } else {
            sc.putParameter(StringParameter.getInstance(FPU_MASK_PROP.getName(), getFPUnitCustomMask()));
            sc.putParameter(DefaultParameter.getInstance(CUSTOM_SLIT_WIDTH.getName(), getCustomSlitWidth()));
        }

When using a custom mask, for some reason we don't add an FPU property with the value `CUSTOM_MASK` as you might expect.  Rather than change that and risk breaking something downstream I decided to update `GmosRule.getFPU` to return `CUSTOM_MASK` in this case.  The other idea would be to explicitly check for `null` as we'd done in the past, but I do think it is reasonable in the context of Phase 2 Checking at least to expect a disperser and an FPU in the sequence.

Someone else should take a close look at this.