Fix building with no MPI types enabled

If both mpich and openmpi are not used (via --without options) the build
fails.  The list of mpi types is undefined in this case, and it is not
referenced correctly to be treated as an empty list.

Signed-off-by: Trent Piepho <tpiepho@gmail.com>

fftw.spec

@@ -329,7 +329,7 @@ for ((iprec=0; iprec<%{nprec}; iprec++)) ; do
 done
 
 # MPI Builds - this duplicates the non-mpi builds, but oh well
-for mpi in %{mpi_list} ; do
+for mpi in %{?mpi_list} ; do
     module load mpi/${mpi}-%{_arch}
     # Loop over precisions - no quad precision support with MPI
     for((iprec=0;iprec<%{nmpiprec};iprec++)) ; do
@@ -367,7 +367,7 @@ for((iprec=0;iprec<%{nprec};iprec++)) ; do
 done
 
 # MPI
-for mpi in %{mpi_list} ; do
+for mpi in %{?mpi_list} ; do
     module load mpi/${mpi}-%{_arch}
     for((iprec=0;iprec<%{nmpiprec};iprec++)) ; do
         %make_install -C ${mpi}-${prec_name[iprec]}
@@ -396,7 +396,7 @@ done
 # MPI
 # Allow oversubscription with openmpi
 export OMPI_MCA_rmaps_base_oversubscribe=1
-for mpi in %{mpi_list} ; do
+for mpi in %{?mpi_list} ; do
     module load mpi/${mpi}-%{_arch}
     for((iprec=0;iprec<%{nmpiprec};iprec++)) ; do
         export LD_LIBRARY_PATH=$bdir/${prec_name[iprec]}/.libs:$bdir/${prec_name[iprec]}/threads/.libs
@@ -529,6 +529,7 @@ done
 - Enable NEON on aarch64
 - Clean up precision list
 - Fix for OpenMPI build with < 4 processors
+- Fix building with no enabled MPI types
 
 * Thu Mar 02 2023 Orion Poplawski <orion@nwra.com> - 3.3.10-5
 - Use make macros