Do not add an empty MetaData call

[gordonwatts]

• Empty MetaData calls are illegal - and cause a backend crash
• The QMetaData, if called first, can generate an empty metadata call
• Prevent this by properly creating a new empty ast

[codecov-commenter]

Codecov Report

Merging #126 (8f71bab) into master (2e4b59e) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   96.77%   96.79%   +0.02%     
==========================================
  Files          15       15              
  Lines        1272     1281       +9     
==========================================
+ Hits         1231     1240       +9     
  Misses         41       41              

Flag	Coverage Δ
unittests	96.79% <100.00%> (+0.02%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
func_adl/object_stream.py	99.00% <100.00%> (+0.06%)	:arrow_up:
func_adl/util_types.py	97.89% <100.00%> (+0.06%)	:arrow_up: