Fix pack builder suggest panic

[yedamao]

• check inspector.InspectBuilder returned info is not nil

Summary

Fix pack builder suggest panic

Output

Before

go test -v --run=Test_getBuilderDescription
=== RUN   Test_getBuilderDescription
--- FAIL: Test_getBuilderDescription (0.00s)
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=0x8 pc=0x171447c]

goroutine 6 [running]:
testing.tRunner.func1.2(0x185ed40, 0x1ecf140)
        /usr/local/Cellar/go/1.16.6/libexec/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc00039a600)
        /usr/local/Cellar/go/1.16.6/libexec/src/testing/testing.go:1146 +0x4b6
panic(0x185ed40, 0x1ecf140)
        /usr/local/Cellar/go/1.16.6/libexec/src/runtime/panic.go:965 +0x1b9
github.com/buildpacks/pack/internal/commands.getBuilderDescription(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1a5d340, 0x1f15870, 0x3b20f68, 0x167da3c04692c)
        /Users/damao/workspace/contribute/pack/internal/commands/suggest_builders.go:121 +0x9c
github.com/buildpacks/pack/internal/commands.Test_getBuilderDescription(0xc00039a600)
        /Users/damao/workspace/contribute/pack/internal/commands/stack_suggest_test.go:77 +0x4b
testing.tRunner(0xc00039a600, 0x19a5658)
        /usr/local/Cellar/go/1.16.6/libexec/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
        /usr/local/Cellar/go/1.16.6/libexec/src/testing/testing.go:1238 +0x2b3
exit status 2
FAIL    github.com/buildpacks/pack/internal/commands    0.569s

After

go test -v --run=Test_getBuilderDescription
=== RUN   Test_getBuilderDescription
--- PASS: Test_getBuilderDescription (0.00s)
PASS
ok      github.com/buildpacks/pack/internal/commands    1.001s

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #1250

[codecov]

Codecov Report

Merging #1255 (4d2d145) into main (f962404) will decrease coverage by 1.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
- Coverage   81.23%   80.09%   -1.14%     
==========================================
  Files         140      140              
  Lines        8581     8581              
==========================================
- Hits         6970     6872      -98     
- Misses       1176     1274      +98     
  Partials      435      435              

Flag	Coverage Δ
os_linux	79.81% <100.00%> (-0.05%)	⬇️
os_macos	77.16% <100.00%> (ø)
os_windows	79.95% <100.00%> (-1.17%)	⬇️
unit	80.09% <100.00%> (-1.14%)	⬇️

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

[jromero]

My initial inclination was that there's something more hidden behind the covers here. After looking at the implementation it does look like both err and info may be nil in the "Not Found" scenario. This patch will fix the presented error but I do wonder if a better approach would be to return our own error of "ErrNotFound" like most other common apis. 🥫 for thought.

[jromero]

Thank you @yedamao for the contribution! Much appreaciated!

[yedamao]

I noticed the image.ErrNotFound error, but not sure that should be returned.