tgc-revival: Fix Compute Instance PMU round-trip serialization in TGC Next

[zli82016]

Resolves integration test failure TestAccComputeInstance_performanceMonitoringUnit by defaulting empty performance_monitoring_unit to STANDARD during tfplan2cai and safeguarding empty map flattening in cai2hcl.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit ff633b7:

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terraform-google-conversion	View Diff	2 files changed, 10 insertions(+), 2 deletions(-)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit e65d265:

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Your PR generated the following diffs in downstream repositories:

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terraform-google-conversion	View Diff	3 files changed, 10 insertions(+), 3 deletions(-)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 8135b92:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
terraform-google-conversion	View Diff	4 files changed, 14 insertions(+), 6 deletions(-)