fix(upgrade): fix registerForNg1Tests

[jpvanhal]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

When trying the example in for UpgradeAdapter#registerForNg1Tests, it crashes with the following exception:

Unhandled Promise rejection: Cannot read property 'get' of null ; Zone: ProxyZone ; Task: Promise.then ; Value: TypeError: Cannot read property 'get' of null
    at UpgradeAdapterRef._bootstrapDone (base/dist/all/@angular/upgrade/src/upgrade_adapter.js:697)
    at base/dist/all/@angular/upgrade/src/upgrade_adapter.js:315
    at ZoneDelegate.invoke (zone.js:229)
    at ProxyZoneSpec.onInvoke (proxy.js:79)
    at ZoneDelegate.invoke (zone.js:228)
    at Zone.run (zone.js:113)
    at zone.js:509
    at ZoneDelegate.invokeTask (zone.js:262)
    at ProxyZoneSpec.onInvokeTask (proxy.js:103)
    at ZoneDelegate.invokeTask (zone.js:261)
    at Zone.runTask (zone.js:151)
    at drainMicroTaskQueue (zone.js:405)
    at XMLHttpRequest.ZoneTask.invoke (zone.js:336) TypeError: Cannot read property 'get' of null
    at UpgradeAdapterRef._bootstrapDone (http://localhost:9878/base/dist/all/@angular/upgrade/src/upgrade_adapter.js:697:40)
    at http://localhost:9878/base/dist/all/@angular/upgrade/src/upgrade_adapter.js:315:70
    at ZoneDelegate.invoke (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:229:26)
    at ProxyZoneSpec.onInvoke (http://localhost:9878/base/node_modules/zone.js/dist/proxy.js:79:39)
    at ZoneDelegate.invoke (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:228:32)
    at Zone.run (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:113:43)
    at http://localhost:9878/base/node_modules/zone.js/dist/zone.js:509:57
    at ZoneDelegate.invokeTask (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:262:35)
    at ProxyZoneSpec.onInvokeTask (http://localhost:9878/base/node_modules/zone.js/dist/proxy.js:103:39)
    at ZoneDelegate.invokeTask (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:261:40)
    at Zone.runTask (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:151:47)
    at drainMicroTaskQueue (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:405:35)
    at XMLHttpRequest.ZoneTask.invoke (http://localhost:9878/base/node_modules/zone.js/dist/zone.js:336:25)

This is because upgrade.ng1Injector passed to _bootstrapDone in registerForNg1Tests is null.

What is the new behavior?

UpgradeAdapter#registerForNg1Tests doesn't throw the above exception anymore.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

Fix an issue in registerForNg1Tests, where it passes a null as
ng1Injector to _bootstrapDone. This causes a "TypeError: Cannot
read property 'get' of null" to be thrown from _bootstrapDone.

[petebacondarwin]

Good stuff @jpvanhal - thanks for this fix.

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