fix issue in converting value to nil when calling IsNil return ture for gdb.Core

[niluan304]

// iString is the type assert api for String.
type iString interface {
	String() string
}

// iNil if the type assert api for IsNil.
type iNil interface {
	IsNil() bool
}

check whether value implements the iNil interface, before iString interface.

fix issue in converting gjson.New('') and gjson.New(nil) to nil, related Issues: #2719

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (c5c2938) 78.88% compared to head (27e1092) 78.90%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2720      +/-   ##
==========================================
+ Coverage   78.88%   78.90%   +0.01%     
==========================================
  Files         632      632              
  Lines       51880    51885       +5     
==========================================
+ Hits        40926    40939      +13     
+ Misses       8927     8924       -3     
+ Partials     2027     2022       -5     

Flag	Coverage Δ
go-1.15-386	78.92% <100.00%> (+<0.01%)	⬆️
go-1.15-amd64	78.92% <100.00%> (+0.02%)	⬆️
go-1.16-386	78.93% <100.00%> (+0.04%)	⬆️
go-1.16-amd64	78.91% <100.00%> (+0.02%)	⬆️
go-1.17-386	78.93% <100.00%> (+0.01%)	⬆️
go-1.17-amd64	78.91% <100.00%> (-0.01%)	⬇️
go-1.18-386	78.84% <100.00%> (-0.02%)	⬇️
go-1.18-amd64	78.83% <100.00%> (+<0.01%)	⬆️
go-1.19-386	78.91% <100.00%> (-0.01%)	⬇️
go-1.19-amd64	78.92% <100.00%> (+<0.01%)	⬆️
go-1.20-386	78.81% <100.00%> (-0.04%)	⬇️
go-1.20-amd64	78.88% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
database/gdb/gdb_func.go	86.84% <ø> (+0.50%)	⬆️
database/gdb/gdb_core_structure.go	63.51% <100.00%> (+2.11%)	⬆️

... and 1 file with indirect coverage changes

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[gqcn]

@niluan304 It's awesome! Thanks for your contribution!