neotest: don't collect coverage for contracts with empty DI

[AnnaShaleva]

Users are allowed to provide any contract to neotest, including those contracts that don't have DebugInfo filled in. Neotest should filter them out, otherwise panic may occur on attempt to write coverage profile:

--- FAIL: TestDeploys (0.01s)
panic: runtime error: invalid memory address or nil pointer dereference
	panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0xc6f5b5]

goroutine 1735 [running]:
testing.tRunner.func1.2({0xd7aa80, 0x1723f60})
	/usr/local/go/src/testing/testing.go:1631 +0x24a
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1634 +0x377
panic({0xd7aa80?, 0x1723f60?})
	/usr/local/go/src/runtime/panic.go:770 +0x132
github.com/nspcc-dev/neo-go/pkg/neotest.processCover()
	/home/anna/go/pkg/mod/github.com/nspcc-dev/[email protected]/pkg/neotest/coverage.go:143 +0xd5
github.com/nspcc-dev/neo-go/pkg/neotest.writeCoverageReport({0x10c6260, 0xc00033a378})
	/home/anna/go/pkg/mod/github.com/nspcc-dev/[email protected]/pkg/neotest/coverage.go:123 +0x4b
github.com/nspcc-dev/neo-go/pkg/neotest.reportCoverage({0x10db278, 0xc00021a000})
	/home/anna/go/pkg/mod/github.com/nspcc-dev/[email protected]/pkg/neotest/coverage.go:118 +0x165
github.com/nspcc-dev/neo-go/pkg/neotest.(*Executor).trackCoverage.func1()
	/home/anna/go/pkg/mod/github.com/nspcc-dev/[email protected]/pkg/neotest/basic.go:182 +0x1b
testing.(*common).Cleanup.func1()
	/usr/local/go/src/testing/testing.go:1175 +0x10f
testing.(*common).runCleanup(0xc00021a000, 0x58d7c0?)
	/usr/local/go/src/testing/testing.go:1353 +0xdb
testing.(*common).runCleanup.func2()
	/usr/local/go/src/testing/testing.go:1337 +0x47
panic({0xd7aa80?, 0x1723f60?})

An example of such partially-filled contract is:
https://github.com/nspcc-dev/neofs-contract/blob/a8d5e001a2e5a552676eecd40ef78629ecdfafa2/tests/deploys_test.go#L13 https://github.com/nspcc-dev/neofs-contract/blob/a8d5e001a2e5a552676eecd40ef78629ecdfafa2/contracts/contracts.go#L96

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.20%. Comparing base (1115193) to head (7fac3bc).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/neotest/coverage.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3600      +/-   ##
==========================================
- Coverage   85.25%   85.20%   -0.06%     
==========================================
  Files         333      333              
  Lines       39007    39009       +2     
==========================================
- Hits        33256    33238      -18     
- Misses       4179     4205      +26     
+ Partials     1572     1566       -6     

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