[Bug]: Forced merge precludes using modules without joinin keys

### What happened?

Even in modules that treat datasets independently, `teal.transform` attempts to join them and fails if no keys are provided.

modified example app

```
library(teal)
library(teal.modules.general)

data <- teal_data()
data <- within(data, {
require(nestcolor)
USArrests <- USArrests
iris <- iris
})

app <- init(
data = data,
modules = modules(
tm_a_pca(
"PCA",
dat = list(
data_extract_spec(
dataname = "USArrests",
select = select_spec(
choices = variable_choices(
data = data[["USArrests"]], c("Murder", "Assault", "UrbanPop", "Rape")
),
selected = c("Murder", "Assault"),
multiple = TRUE
),
filter = NULL
),
data_extract_spec(
dataname = "iris",
select = select_spec(
choices = variable_choices(
data = data[["iris"]], function(data) names(Filter(is.numeric, data))
),
selected = NULL,
multiple = TRUE
),
filter = NULL
)
)
)
)
)
if (interactive()) {
shinyApp(app$ui, app$server)
}
```


### sessionInfo()

```R
> packageVersion("teal.transform")
[1] ‘0.5.0’
> packageVersion("teal.modules.general")
[1] ‘0.3.0’
```

### Relevant log output

```R

```

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