left(), right(): Remove NULL from valid 2nd arg types

[nafraf]

This patch is to solve this small bug:

GRAPH.QUERY g "RETURN left('abc', 0.5)"
(error) Type mismatch: expected Integer or Null but was Float
GRAPH.QUERY g "RETURN left('abc', null)"
(error) length must be a non-negative integer

The reply to the first call states that Null is a valid type for the second parameter, while actually it is not.
Same for right().
When passing Null we should receive Type mismatch: expected Integer.

After applying the patch:

GRAPH.QUERY g "RETURN left('abc', 0.5)"
(error) Type mismatch: expected Integer but was Float
GRAPH.QUERY g "RETURN left('abc', null)"
(error) Type mismatch: expected Integer but was Null

[AviAvni]

now need to fix the function itself

[codecov]

Codecov Report

Patch coverage: 83.33% and no project coverage change.

Comparison is base (fb9e46c) 90.34% compared to head (2b8a01a) 90.34%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2966   +/-   ##
=======================================
  Coverage   90.34%   90.34%           
=======================================
  Files         283      283           
  Lines       28042    28044    +2     
=======================================
+ Hits        25334    25336    +2     
  Misses       2708     2708           

Impacted Files	Coverage Δ
src/arithmetic/string_funcs/string_funcs.c	96.85% <83.33%> (-0.41%)	⬇️

... and 4 files with indirect coverage changes

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