fix: Add nil check to support bundle printer that prints remotecfg & local sources

[dehaansa]

PR Description

@mattdurham encountered the following panic when testing the RC for 1.6

EDIT: Also removing public preview requirement for support bundle.

2025/01/08 12:10:50 http: panic serving 127.0.0.1:60176: runtime error: invalid memory address or nil pointer dereference
goroutine 1288 [running]:
net/http.(*conn).serve.func1()
        /home/mdurham/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:1947 +0xbe
panic({0x9456080?, 0x114afc70?})
        /home/mdurham/go/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:785 +0x132
github.com/grafana/alloy/syntax/printer.(*walker).walkFile(0xc001d2f1f8, 0x0)
        /home/mdurham/source/alloy_grafana/vendor/github.com/grafana/alloy/syntax/printer/walker.go:35 +0x25
github.com/grafana/alloy/syntax/printer.(*walker).Walk(0x10?, {0xbc0bee0?, 0x0?})
        /home/mdurham/source/alloy_grafana/vendor/github.com/grafana/alloy/syntax/printer/walker.go:20 +0x45
github.com/grafana/alloy/syntax/printer.(*Config).Fprint(0xa9ae0a0?, {0xbc0be40, 0xc00dcf5350}, {0xbc0bee0, 0x0})
        /home/mdurham/source/alloy_grafana/vendor/github.com/grafana/alloy/syntax/printer/printer.go:29 +0x169
github.com/grafana/alloy/internal/service/http.printFileRedacted(0x0)
        /home/mdurham/source/alloy_grafana/internal/service/http/http.go:670 +0x56
github.com/grafana/alloy/internal/service/http.remoteCfgRedactedCachedConfig({0xbcd6920?, 0xc001b4ae00?})
        /home/mdurham/source/alloy_grafana/internal/service/http/http.go:660 +0x52
github.com/grafana/alloy/internal/service/http.(*Service).Run.(*Service).generateSupportBundleHandler.func12({0xbca82f0, 0xc001a1c9c0}, 0xc0009dac80)
        /home/mdurham/source/alloy_grafana/internal/service/http/http.go:341 +0x2ea
net/http.HandlerFunc.ServeHTTP(0xbc9b500?, {0xbca82f0?, 0xc001a1c9c0?}, 0xa9b615a?)
        /home/mdurham/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:2220 +0x29
go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux.traceware.ServeHTTP({{0xa998833, 0x5}, {0xbc3b528, 0xc00119f680}, {0xbc9df30, 0xc001432340}, {0xbc0c5e0, 0xc001d12d80}, 0xafc8af8, 0x0, ...}, ...)
        /home/mdurham/source/alloy_grafana/vendor/go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux/mux.go:179 +0xb91
github.com/gorilla/mux.(*Router).ServeHTTP(0xc0010d29c0, {0xbc9b500, 0xc001b50380}, 0xc0009da640)
        /home/mdurham/source/alloy_grafana/vendor/github.com/gorilla/mux/mux.go:212 +0x1e2
golang.org/x/net/http2/h2c.h2cHandler.ServeHTTP({{0xbc0d0e0?, 0xc0010d29c0?}, 0xc0005c80e0?}, {0xbc9b500, 0xc001b50380}, 0xc0009da640)
        /home/mdurham/source/alloy_grafana/vendor/golang.org/x/net/http2/h2c/h2c.go:125 +0x673
net/http.serverHandler.ServeHTTP({0xc00dcf5110?}, {0xbc9b500?, 0xc001b50380?}, 0x6?)
        /home/mdurham/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:3210 +0x8e
net/http.(*conn).serve(0xc03d3a2510, {0xbcc4ac8, 0xc001b84300})
        /home/mdurham/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:2092 +0x5d0
created by net/http.(*Server).Serve in goroutine 722
        /home/mdurham/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:3360 +0x485

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

[github-actions]

💻 Deploy preview deleted.

[mattdurham]

Lgtm will wait for Clayton’s blessing

[mattdurham]

Gonna go ahead and merge this since it was only a label change on the docs.