fix: add idle handler offset nil check

[jy4096]

Add idle handler offset nil check to fix the panic.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x1d493cc]
goroutine 369 [running]:
github.com/numaproj/numaflow/pkg/watermark/publish.(*publish).PublishIdleWatermark(0xc000598120, {0xc00064c640?, 0x41?, 0x3bb0be0?}, {0x0, 0x0}, 0x1)
	/home/runner/work/numaflow/numaflow/pkg/watermark/publish/publisher.go:207 +0x16c
github.com/numaproj/numaflow/pkg/shared/idlehandler.PublishIdleWatermark({0x294e5e8, 0xc0007dc300}, {0x294ed20, 0xc000222310}, {0x294eee0, 0xc000598120}, {0x294ef18, 0xc00065eb60}, 0xc00061e0a0?, {0x258897f, ...}, ...)
	/home/runner/work/numaflow/numaflow/pkg/shared/idlehandler/idlehandler.go:62 +0x645
github.com/numaproj/numaflow/pkg/udf/forward.(*InterStepDataForward).forwardAChunk(0xc00001a8c0, {0x294e5e8?, 0xc0007dc300})
	/home/runner/work/numaflow/numaflow/pkg/udf/forward/forward.go:381 +0x1ec7
github.com/numaproj/numaflow/pkg/udf/forward.(*InterStepDataForward).Start.func1()
	/home/runner/work/numaflow/numaflow/pkg/udf/forward/forward.go:156 +0xf6
created by github.com/numaproj/numaflow/pkg/udf/forward.(*InterStepDataForward).Start
	/home/runner/work/numaflow/numaflow/pkg/udf/forward/forward.go:135 +0xca

Also update the goroutine in map_udf.go Start function