impl: check if options are empty

[dbolduc]

Part of the work for #7688

Add an internal helper to see if Options are empty or not.

There are two code paths in the bigtable::Table client. One that uses client_ and one that uses connection_. Per operation options only apply to the path that uses the connection_. I do not want to check if opts.has<T> for each T option. So, we just check to see if the options are empty.

The code in bigtable will look vaguely like this:

Status Table::Foo(Options opts) {
  if (!connection_ && !google::cloud::internal::IsEmpty(opts)) {
    return Status(StatusCode::kFailedPrecondition,
                   "Per-operation options only apply to `Table`s constructed "
                   "with a `DataConnection`.");
  }
  // etc...
}

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9617 (f49f082) into main (7735b70) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #9617      +/-   ##
==========================================
- Coverage   94.36%   94.36%   -0.01%     
==========================================
  Files        1491     1491              
  Lines      137590   137599       +9     
==========================================
+ Hits       129842   129848       +6     
- Misses       7748     7751       +3     

Impacted Files	Coverage Δ
google/cloud/options.h	97.82% <ø> (ø)
google/cloud/options.cc	100.00% <100.00%> (ø)
google/cloud/options_test.cc	100.00% <100.00%> (ø)
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.34%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.77% <0.00%> (+0.07%)	⬆️

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