[credential-helper] Ignore null values in JSON response

[Yannic]

This protects against GetCredentialsResponses of the form {"expires": null} or {"headers": null}.

Seen in the wild:

Caused by: com.google.devtools.build.lib.authandtls.credentialhelper.CredentialHelperException: Failed to get credentials for '<redacted>' from helper '<redacted>': error parsing output. stderr:
	at com.google.devtools.build.lib.authandtls.credentialhelper.CredentialHelper.getCredentials(CredentialHelper.java:147)
	at com.google.devtools.build.lib.authandtls.credentialhelper.CredentialHelperCredentials.getCredentialsFromHelper(CredentialHelperCredentials.java:94)
	at com.github.benmanes.caffeine.cache.BoundedLocalCache.lambda$doComputeIfAbsent$14(BoundedLocalCache.java:2688)
	at java.base/java.util.concurrent.ConcurrentHashMap.compute(Unknown Source)
	at com.github.benmanes.caffeine.cache.BoundedLocalCache.doComputeIfAbsent(BoundedLocalCache.java:2686)
	at com.github.benmanes.caffeine.cache.BoundedLocalCache.computeIfAbsent(BoundedLocalCache.java:2669)
	at com.github.benmanes.caffeine.cache.LocalCache.computeIfAbsent(LocalCache.java:112)
	at com.github.benmanes.caffeine.cache.LocalManualCache.get(LocalManualCache.java:62)
	at com.google.devtools.build.lib.authandtls.credentialhelper.CredentialHelperCredentials.getRequestMetadata(CredentialHelperCredentials.java:66)
	at com.google.auth.Credentials.blockingGetToCallback(Credentials.java:128)
	at com.google.auth.Credentials$1.run(Credentials.java:114)
	... 3 more
Caused by: com.google.gson.JsonSyntaxException: Expected value of 'expires' to be a string, got NULL
	at com.google.devtools.build.lib.authandtls.credentialhelper.GetCredentialsResponse$GsonTypeAdapter.read(GetCredentialsResponse.java:178)
	at com.google.devtools.build.lib.authandtls.credentialhelper.GetCredentialsResponse$GsonTypeAdapter.read(GetCredentialsResponse.java:85)
	at com.google.gson.TypeAdapter$1.read(TypeAdapter.java:308)
	at com.google.gson.Gson.fromJson(Gson.java:1361)
	at com.google.gson.Gson.fromJson(Gson.java:1262)
	at com.google.gson.Gson.fromJson(Gson.java:1199)
	at com.google.devtools.build.lib.authandtls.credentialhelper.CredentialHelper.getCredentials(CredentialHelper.java:133)
	... 13 more

RELNOTES: credential-helper: treat null values in response as unset.