add fix for issue with empty attributes

[rcannood]

This PR adds a fix for the issue described in #208 and tested in #209.

I made changes to several instances XLENGTH(R_buf) == 0 was being used to check R_buf should be cast to a void pointer.

  if(XLENGTH(R_buf) == 0) {
    buf = NULL;
  }
  else {
    buf = (void *) VOIDPTR(R_buf);
  }

However, since the implementation of VOIDPTR is:

void* VOIDPTR(SEXP x) {

  switch(TYPEOF(x)) {
  case REALSXP:
    return((void *) REAL(x));
  case INTSXP:
    return((void *) INTEGER(x));
  case LGLSXP:
    return((void *) LOGICAL(x));
  case CPLXSXP:
    return((void *) COMPLEX(x));
  case RAWSXP:
    return((void *) RAW(x));
  case STRSXP:
    return((void *) STRING_PTR(x));
  case VECSXP:
    return((void *) VECTOR_PTR(x));
  default:
    error("Type cannot be converted to voidptr\n");
  }
}

I figured it may perhaps make more sense to check whether TYPEOF(R_buf) == NILSXP instead. I replaced XLENGTH(.) == 0 with TYPEOF(R_buf) == NILSXP in the places where the unit tests in #209 caused errors. Specifically:

---

If I undo my changes in R_H5Awrite(), I get:

Error (test-h5a.R:226:5): Creating attributes of different types of length 0
Error in `h5attr$write(robj)`: HDF5-API Errors:
    error #000: ../../src/H5A.c in H5Awrite(): line 657: buf parameter can't be NULL
        class: HDF5
        major: Invalid arguments to routine
        minor: Bad value

This may be related to mojaveazure/seurat-disk#15 (comment)

---

If I undo my changes in R_H5Aread(), I get:

Error (test-h5a.R:226:5): Creating attributes of different types of length 0
Error in `h5attr$read()`: HDF5-API Errors:
    error #000: ../../src/H5A.c in H5Aread(): line 702: buf parameter can't be NULL
        class: HDF5
        major: Invalid arguments to routine
        minor: Bad value

---

And if I undo my changes in R_H5Dvlen_reclaim(), I get:

Error (test-h5a.R:247:5): Creating attributes of different types of length 0
Error in `h5attr$read()`: HDF5-API Errors:
    error #000: ../../src/H5Ddeprec.c in H5Dvlen_reclaim(): line 323: invalid argument
        class: HDF5
        major: Invalid arguments to routine
        minor: Bad value

This may be related to #118, PMBio/MuDataSeurat#14 and JiekaiLab/scDIOR#7

---

@hhoeflin I'd appreciate your feedback on this ☺️

[hhoeflin]

Closing as I implemented the fix in #214. Thanks for providing the code.