Fix KINETIC block visitor in case there is an empty statement

[iomaganaris]

Fixes compilation of mod files of dbbs-mod-collection (after small changes in mod files in https://github.com/iomaganaris/dbbs-mod-collection/tree/magkanar/nmodl)

• Move definition of functors after declaration of functions
• Fix issue with parsing () from sympy if ode_rhs is empty (Fixes [dbbs-mod-collection] Issues with KINETIC block #1008)

[codecov]

Codecov Report

Merging #1009 (e4a7cf6) into master (a1a035c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1009   +/-   ##
=======================================
  Coverage   69.98%   69.98%           
=======================================
  Files         188      188           
  Lines       25659    25659           
=======================================
  Hits        17957    17957           
  Misses       7702     7702           

Files Changed	Coverage Δ
src/visitors/kinetic_block_visitor.cpp	96.13% <100.00%> (ø)

[bbpbuildbot]

Logfiles from GitLab pipeline #147025 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ spack_setup
• ✅ build:intel
• ✅ build:nvhpc
• ✅ test:intel
• ✅ test:nvhpc

[iomaganaris]

Should we merge this now @pramodk @cattabiani ?

[cattabiani]

Sorry, I missed the mail. LGTM!