service-router: PrefixRewrite cannot be empty string (and docs are unclear/the example produces confusing behavior)

#### Overview of the Issue

The docs for using PrefixRewrite are not great and it seems like there's a bug that won't allow me to use empty string as a value

1: I don't actually see where in the docs PrefixRewrite is actually clearly explained. I found the doc entry for it but it doesn't explain how it's actually used. [This blog post](https://www.hashicorp.com/blog/hashicorp-consul-1-6) is the only example for PrefixRewrite that I can find, and it does not have the structure that is currently in use (I'm assuming the structure changed since then?)

I did reverse engineer the logic by looking at the [golang tests](https://github.com/hashicorp/consul/blob/a14950025a0135224bab8969ed60328db29365a4/agent/structs/config_entry_discoverychain_test.go#L1615-L1647) that involve PrefixRewrite and I was able to deduce that the PrefixRewrite value is used as the direct replacement value for whatever PathPrefix matched

2: That said, the example on that blog post as well as the golang tests all seem to use an example that works like this:

```
{
"Match": {
"HTTP": {
"PathPrefix": "/sp"
}
},
"Destination": {
"Service": "sp-api",
"Namespace": "testing-rx8",
"PrefixRewrite": "/"
}
},
```

However this results in a request like this: `curl localhost:21001/sp/bar` -> `//bar`

This is confusing. I understand that technically it is valid from consul's POV but it produces strange behavior where the request ends up with an extra slash. The tests should be changed and an example should be added to the documentation page

3: Finally, I think there may actually be a bug, because when I try to produce the expected result by using the following configuration:

```
{
"Match": {
"HTTP": {
"PathPrefix": "/sp"
}
},
"Destination": {
"Service": "sp-api",
"Namespace": "testing-rx8",
"PrefixRewrite": ""
}
},
```

It won't let me use an empty string for PrefixRewrite -- when I read the config back it shows that the PrefixRewrite was simply eliminated:

```
cjohnson@atpplatformconsul2:PRODUCTION:~> cat config.json
{
"Kind": "service-router",
"Name": "api2",
"Namespace": "testing-rx8",
"Routes": [
{
"Match": {
"HTTP": {
"PathPrefix": "/forms"
}
},
"Destination": {
"Service": "forms-api",
"Namespace": "testing-rx8",
"PrefixRewrite": ""
}
}
]
}
cjohnson@atpplatformconsul2:PRODUCTION:~> consul config write config.json
Config entry written: service-router/api2
cjohnson@atpplatformconsul2:PRODUCTION:~> consul config read -namespace=testing-rx8 -kind=service-router -name=api2
{
"Kind": "service-router",
"Name": "api2",
"Namespace": "testing-rx8",
"Routes": [
{
"Match": {
"HTTP": {
"PathPrefix": "/forms"
}
},
"Destination": {
"Service": "forms-api",
"Namespace": "testing-rx8"
}
}
],
"CreateIndex": 646564,
"ModifyIndex": 646564
}
```

### Consul info for both Client and Server

Client info

```
agent:
check_monitors = 0
check_ttls = 0
checks = 87
services = 58
build:
prerelease =
revision = cf701c09
version = 1.9.7+ent
consul:
acl = enabled
known_servers = 10
server = false
license:
customer = 480e18da-6001-8ce2-7b8e-8642e1cb3225
expiration_time = 2023-03-25 00:00:00 +0000 UTC
features = Automated Backups, Automated Upgrades, Enhanced Read Scalability, Network Segments, Redundancy Zone, Advanced Network Federation, Namespaces, SSO, Audit Logging
id = eda50725-0f08-ccaf-f3e3-b26cae0812d4
install_id = *
issue_time = 2021-03-25 04:56:39.925162515 +0000 UTC
modules = Global Visibility, Routing and Scale, Governance and Policy
product = consul
start_time = 2021-03-24 00:00:00 +0000 UTC
runtime:
arch = amd64
cpu_count = 4
goroutines = 1017
max_procs = 4
os = linux
version = go1.15.13
serf_lan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 108
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 34097
members = 12
query_queue = 0
query_time = 862
```

Server info

```
agent:
check_monitors = 0
check_ttls = 1
checks = 1
services = 1
build:
prerelease =
revision = cf701c09
version = 1.9.7+ent
consul:
acl = enabled
bootstrap = false
known_datacenters = 4
leader = false
leader_addr = 10.3.52.53:8300
server = true
license:
customer = 480e18da-6001-8ce2-7b8e-8642e1cb3225
expiration_time = 2023-03-25 00:00:00 +0000 UTC
features = Automated Backups, Automated Upgrades, Enhanced Read Scalability, Network Segments, Redundancy Zone, Advanced Network Federation, Namespaces, SSO, Audit Logging
id = eda50725-0f08-ccaf-f3e3-b26cae0812d4
install_id = *
issue_time = 2021-03-25 04:56:39.925162515 +0000 UTC
modules = Global Visibility, Routing and Scale, Governance and Policy
product = consul
start_time = 2021-03-24 00:00:00 +0000 UTC
raft:
applied_index = 646355
commit_index = 646355
fsm_pending = 0
last_contact = 41.753905ms
last_log_index = 646355
last_log_term = 7804
last_snapshot_index = 629975
last_snapshot_term = 7796
latest_configuration = [{Suffrage:Nonvoter ID:53fca35b-cd1a-0800-b6ee-c60527ca7313 Address:10.3.52.41:8300} {Suffrage:Voter ID:0ac1c505-c157-220a-8d8a-4af254d3d4f0 Address:10.3.52.42:8300} {Suffrage:Voter ID:c2498167-5c9c-18e0-be21-4595f7a20ae4 Address:10.3.52.55:8300} {Suffrage:Nonvoter ID:4a1ae97c-4e4f-be48-19b7-bc92b3742e20 Address:10.3.52.56:8300} {Suffrage:Voter ID:64e0f24d-a51f-c82e-17aa-2eb20c165617 Address:10.3.52.14:8300} {Suffrage:Nonvoter ID:c3809b2c-e13a-662f-03da-7f7eccd04edb Address:10.3.52.52:8300} {Suffrage:Nonvoter ID:769747f8-e741-25e1-f5f3-4714706db717 Address:10.3.52.35:8300} {Suffrage:Voter ID:107fc5ce-dbf6-1b5f-2cfa-263608ec691b Address:10.3.52.53:8300} {Suffrage:Nonvoter ID:f5ef8ba0-a971-2aa6-1eb8-d0e0e034f5ac Address:10.3.52.54:8300} {Suffrage:Voter ID:0ee8fe98-540f-b67b-878c-1b50f91a9d09 Address:10.3.52.36:8300}]
latest_configuration_index = 0
num_peers = 0
protocol_version = 3
protocol_version_max = 3
protocol_version_min = 0
snapshot_version_max = 1
snapshot_version_min = 0
state = Follower
term = 7804
runtime:
arch = amd64
cpu_count = 2
goroutines = 285
max_procs = 2
os = linux
version = go1.15.13
serf_lan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 24
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 1098
members = 10
query_queue = 0
query_time = 1
serf_wan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 1
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 3766
members = 33
query_queue = 0
query_time = 862
```

Start with agent/structs/config_entry_discoverychain_test.go, especially the PrefixRewrite cases linked in the issue, and inspect the service-router configuration and documentation entry they describe. Verify how an empty PrefixRewrite is stored and how replacing /sp with / affects /sp/bar; done means the behavior is covered by updated tests and the documented example is unambiguous.

Written by the indexing model from the issue text.