Entry: Emit notify::pixbuf whenever icon's cleared

not just if it was specifically using pixbuf storage type, since the
getter works for any storage type, so in any case when we had storage
and clear it, we need to notify that the effective :pixbuf is now NULL.
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passed #169699
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00:06:29

manual #169700
win32 allowed to fail manual
msys2-mingw32